Vista 64bit and Cryptographic Services

So, my brother buys a new computer over the summer.

AMD Phenom 9600 Quad-core
4 Gigs Ram
Vista Home Premium SP1 64-bit

In the last couple of weeks, it has gotten really slow and, upon examination, I found that he has a copy of svchost.exe using ~25% of the processor (i.e. 1 core) and generally making things crawl. Looking closer, I find that it is the Cryptographic Services that is doing this. Kill it, things go back to normal. Start it, it jump immediately to 25%. I've looked into the possibility that it was Windows Update causing problems, tried the stopping, registering its dlls again, restarting it solution and I've tried getting rid of the update download directory, but it doesn't seem to have any effect. Tried booting up with no non-Microsoft services running - same issue. It doesn't seem to be a virus or spyware either. He runs AVG (home edition) and Spybot and neither pick up anything.

At this point, I'm looking for any suggestions of what to try. Thanks.

Scott

Yes, I fixed it by nuking the computer and starting again from scratch. I spent hours on it but got nowhere. It didn't seem like a misbehaving program or a virus so it might have been an update gone bad or a corrupt file, but I've got nothing more than that. Sorry.