Yes... sort of
Bonuses to an ability score count towards your total score, which is what prerequisites care about. There rules do not make much of a distinction between your base score and your augmented score.
The problem is that for very-temporary bonuses, this gets pretty cheesy really fast. With feats, it’s probably not so bad, because you just lose the ability to actually use the feat when the bonus wears off, but with Prestige Classes, that doesn’t apply: once in, you can keep using it, unless it comes from Complete Arcane or Complete Warrior.1 That means, potentially, you can qualify for high-power Prestige Classes with only temporary outlays for their prerequisites. Though, in most cases, you can make many of these things permanent with abuses of polymorph any object and/or psychic reformation.
The other issue is that you have to convince your DM that the bonus was in place during level-up, which is kind of awkward for temporary bonuses. A continuous magic item? OK, better not lose that item. Bull’s strength? That’s... kind of unlikely. Why would your character have cast that spell (or convinced someone to cast it on him) at some point in the middle of the night?
The real answer: Talk to your DM
There are almost no cases where a DM is going to just “accept” these kinds of things without forewarning. Talk to your DM. Usually this kind of thing is done because you cannot reasonably meet the prerequisites: try to convince him to just waive them rather than cheese your way around them. 3.x is full of poorly-designed feats and prestige classes that require way too many things to get. Combat Expertise is definitely one such example. You might be able to convince him on this.
About MAD and SAD and why a DM might waive these prerequisites
The real issue is that, early on in D&D, the designers wanted to try to make sure that melee types didn’t go “all Strength” because they thought this would be overpowered. What they didn’t realize is that because of the very-considerable HP inflation between 2nd and 3rd edition, going “all Strength” is almost required to make headway against a lot of monsters. Moreover, in their efforts to make spells other than the obvious blasty ones more interesting, they grossly overpowered the spellcasters.
The result is a situation where the Fighter needs Str and Con maxed, cannot dump Dex or Int, and now he’s looking at four different ability scores he has to divide himself between. The Monk is even worse; he needs everything but Cha.
The Wizard? Max Int, put anything else that can’t go towards Int into Con and Dex, call it a day. Being low on Con or Dex is not the end of the world so long as you’ve got the Int.
These are referred to as “Multiple Ability Dependence” and “Single Ability Dependence” – Fighters need two high and two not-low, Wizards just need one high, and it’d be nice if two others were not-low but he can deal with it if he needs to. These are really serious, systemic problems, and the system overall handles Multiple Ability Dependence very poorly, because you only gain 5 points to put towards ability scores, magic items to improve ability scores get very expensive when you need four of them, and Fighters rely heavily on other, also-very-expensive magic items (read: weapons, armor, wondrous items).
So a DM who understands all of this may be sympathetic about your difficulties with Combat Expertise. If he’s not, he’s unlikely to be suddenly convinced by rules-lawyering about temporary bonuses.
About Power Attack rather than Combat Expertise
Power Attack, obviously enough, does not have much to do with the above, since it requires Strength, not Intelligence. This was more a “hint” by the designers: if you don’t meet this prerequisite easily, you probably don’t actually want this feat, or to even be in this class. Fighters should have high Strength; it’s their most important score. A Dex-based Fighter is possible, but performs relatively poorly; Rogue works much better.
If the character has Str 12, what else does he have? Fighter does not seem like the class for him.
Alternatives: Bonus Feats!
Combat Expertise: Barbarian Totems and Monk Styles
Combat Expertise, as a feat, sucks. You don’t really want it; you want the feats that require it, mostly Improved Trip.2 You can sometimes get these feats as bonus feats without prerequisites.
For example, a Barbarian 2 with the Wolf Totem gets Improved Trip for free, without prerequisites, and that’s on top of Rage or Whirling Frenzy (extra attack instead of Con bonus) or Ferocity (Dex bonus instead of Con bonus) or Mountain Rage (Races of Stone, Goliath-only, become Large while Raging), all of which are excellent for tripping. If you have Complete Champion, the Lion Spirit Totem can also get you Pounce at Barbarian 1 (instead of Fast Movement), which is excellent for any melee character.
I really cannot stress enough how much I recommend Barbarian for this, but if you will not use it, there is also the Monk of the Passive Way, which gets Improved Trip at second level. Barbarian is far, far better than Monk overall, but two levels of Monk is not awful.
There are probably more such options available, these are just the most commonly available ones.
Power Attack: if you really must
A Monk 1 of the Overwhelming Attack style gets Power Attack as a bonus feat without needing to meet the prerequisites. He cannot actually use it because he has BAB +0 (argh, really, WotC?), but he’d have it. Monk 2 for Evasion and Improved Bull Rush might not be a bad choice. Wouldn’t go beyond that if you paid me. Also, a low-Strength Monk is still not a good idea.
1 These two books (and only these two) specify that you lose the benefits of Prestige Classes when you lose the prerequisites; this cannot, by the Errata rules, retroactively apply to all Prestige Classes, and there are in fact problems when you try to do so, e.g. Dragon Disciple
2 Disarming is mediocre, and feinting is really bad, which makes the Improved versions of those feats ill-advised. Whirlwind Attack is an outright trap. Improved Trip is one of the best Core feats available to a Fighter, however, and Combat Reflexes + Improved Trip + Guisarme (or Spiked Chain) is a pretty standard Core build.