Athlon 2600+ Barton is running at 1150Mhz

Hi, I have an Athlon 2600+ that is only running at 1150mhz; when I check the bios, it's set at a 12.5 multiplier, but it's only running with a 100mhz FSB instead of the 166 it's supposed to; when I try to change it to 166, it changes successfully, but the system won't even boot all the way to windows before freezing. I've tried all sorts of things, but I just can't seem to make this work. If anybody has some insight into my dilemma, I'm at my wit's end.

Athlon 2600+ processor
KT4AV MSI mobo
2 GB PC3200 DDR
WinXP SP3


Thank you for any and all help.

Was it working correctly before, or did you make changes that may have caused this to happen?

your FSB speed is set incorrectly. go into BIOS and set the FSB to the correct speed (probably 133). This happens if you have reset the BIOS or your BIOS battery has gone flat.

Well, I noticed it when I was assessing what sorts of upgrades I wanted to do; I realized that the cpu on this machine was woefully underperforming compared to what it's supposed to. I recently added 2GB of RAM, but I noticed the problem before I did that. (I suppose in retrospect it's a bad idea to throw money at something that may not be fixable)

And I've set the FSB to 166 in the bios (which makes the computer freeze while booting) and I've tried 133 as well, which makes it unstable. I've replaced the CMOS battery, cleared it with the jumper, and tried several different settings in the bios, all to no avail. I even tried downloading Fuzzy logic 4 (msi's overclocking tool for dummies, I guess) and incrementally increasing the FSB to failure, and it didn't even go near it's 166 spec.

Is it possible or probable that either the mainboard or the cpu are starting to become faulty? The computer has been in steady use for about 8 years now.

Any input will help. Thank you.