Sudden unexplained Vista problem

I have a Dell dimension 9200 in my office, running Vista, that has happily been running SL for almost two years.

Suddenly this week, when I log in it's as if I have massive lag.. stats window shows stupid network and ping times and it's unusuable (you move about 30 seconds after pressing a key).

1/ I have cleared cache
2/ I have uninstalled all versions of SL and reinstalled latest version
3/ I have not knowingly updated anything (so I suspect there may have been a Vista update that could have caused this?)
4/ the other computer in my office is fine
5/ I can still run CounterStrikeource perfectly on it so it's not a network problem
6/ The partition used for automatic backup is nearly full... but I have turned that off with no effect.
7/ Same problem with virus software disabled

Any ideas? Has anyone else experienced a sudden problem?

System specs?

Have you tried updating your drivers, including your network card driver? Also, try resetting your modem and router. How about rolling back your machine to a restore point from before the problem started?