Ping problems can be difficult to track down, but they boil down to two basic issues:
*) Something on your computer making things slower
*) Something on the network blocking or slowing down traffic
You did a virus scan, that's good. But have you added any other software around the time you noticed the problem? Running any download programs (bittorrent, etc)? Streaming music? What about something that just loads down your computer? Check to see if the CPU is busy, and then see what program is eating it all. Try killing that and checking your ping in Tremulous again.
As for the network, there's again many factors. Are you using wireless instead of a wired connection? Is someone else on the network running Bittorrent or doing a lot of stuff with Youtube? If you can do it, disconnect the other machines from the network leaving only your Tremulous machine and see if your ping improves. If not, the last possibility - which is completely out of your control most of the time - is a network bottleneck with your service provider. The only thing you can do is to try to contact them and have them check your lines (cable/dsl/whatever) and see if they can find a problem. You can try things like "traceroute" to see if you can locate a point in the network stream where packets are getting delayed (I don't know if there's a Windows port of it, but "Mike's traceroute" (mtr) is wonderful for that sort of thing).
