Problem with network

Last week, I was fortunate enough to get the Nimda virus.I was able to remove it after three reboots and virus scans of the machine.Since, then we have had strange things happening to the network.Some mornings, people can not logon to the server. Others are finding that they lose the network connection sometime during the day. The applications running off the server seem to be on a go slow as well.We do not run IIS so I don’t know if we it is virus related.All workstations have been virus checked and are free from any viruses.Server is SBS4.5 with Win98 WS and Office 2000.OUtlook is also continuoulsy hanging on many of the machines.This is only happening to the workstations upstairs.
Could someone please help?