As it appears that setting the FSB to 133 causes the machine to crash, I think leaving it at 100FSB will probably be a good idea until you can get this solved . . . (otherwise, you won't be able to boot to solve it

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What I would try and do is boot into Safe Mode (what OS are you running, by the way?) and see if the problem persists . . . If it does not, then it's something that's loading at startup . . . Enter MSCONFIG (go to Start>Run>MSCONFIG>Enter) and remove the startup items (tab furthest to the right in XP) that might cause the issue. If you don't know what they do, then you can disable all of them. Windows will load what it has to to boot up, and if a program needs something you disabled, it'll start it when it needs it . . . So disabling all of them shouldn't hurt anything.
Also, you might check device manager (Start>Control Panel>System>Third tab on the bottom row>Device Manager) and see if there are any yellow exclamation marks next to any devices. Even if there aren't, you might want to remove the scanner (I assume you have one) from the list by right-clicking and go to remove (or delete . . . I forget which). Then restart, and it should be detected again . . . If that doesn't help, try updating drivers . . .
If none of this helps, then it might be a bigger problem than just software . . . When you took apart the computer, how "apart" was it? If electronics were out, were they in anti-static bags? Did you touch them without grounding yourself? If so, you might have damaged hardware.
One last thing . . . Check the temperature of the processor . . . You might have improperly mounted the heatsink, and it might be overheating . . . if setting it at 100FSB makes it run, that might be keeping it from running as hot . . . Post back if it still won't POST
Good Luck
Z