Sudden appearance of slowdown

Over the last week and a half I have noticed my IE timing out more and more frequently, I thought that it could just be a minor DNS issue with my server so I cleaned up DNS by scavaging and rebuilding the zones.. this didnt help… ive run defrag on the systems on my network that have this issue.. not all seem to have problems.. the 2 main ones with issues are my PIII 933 with 512mb mem and my P4 2.0g with 1024mb memory.. for some odd reason the PII 400 is working fine… I have verified all virus signature files on my network and all are current.. it almost seems to be the same symptoms a co-worker had on his network when he was pummeled by nimda…

Any suggestions?

Ben