Martin Tomes writes about a 20x performance slow-down in checkouts after installing a new anti-virus program. There have been a lot of reports over the years of various anti-virus programs causing slow-down's or other problems with Subversion on Windows. It is a good thing to check if suddenly you are having performance problems or other errors. Hopefully you are at least using a program that lets you fine-tune what and how it is doing the checking.
Another item that people have reported to cause slow-down's is the XP drive indexing. This can be disabled by right-clicking on the drive and choosing Properties. There is an option at the bottom of the General tab that controls whether Indexing is allowed. I do not know if other tools that index your drive, such as the Google Desktop can cause similar problems, but I suspect they can.