£25 is a really good deal for him backing up your files and reinstalling Windows, btw. SP3 doesn't have many issues with AMD/VIA processors that I am aware of. I heard of one when SP3 came out, for I think a specific HP computer, but they fixed it quick.
What I would do if I were you would be to find out which motherboard you have. Which, if you built it, you might know the model but it can't hurt to look again. It might have the name printed on it, or it might not. If not, go http://cpuid.com/pcwizard.php">here
[/L] and install PC Wizard, which should tell you which motherboard you have. Then, depending on who made it (ASUS, Foxconn, Abit, whoever) go to their website, search for downloads on that model, and find the most recent BIOS update, and upgrade to that.
Your computer guy might have installed XP with a disc that already had SP3 on it. That's the only way I know of that it could have SP3, if he didn't connect to the web and do Windows Update.
Phenom II X4 925c2 @ 2.80GHz (95W)
OCZ Platinum PC3-1333 2x2GB 7-7-7-21
Raidmax RX-500AFCheck it out.