If you're on comcast (or any other major broadband provider), if at all possible be behind a router that does NAT. A cheap linksys from one of the bigbox stores will do.
A reinstall from the restore disk wouldnt be a bad idea - especially if there isnt any data needing to be saved.
While doing the restore from disk - dont be connected to the net unless behind a router-based firewall (again, NAT is your friend). before going online (if WinXP), turn on the firewall and immediatly do windows update/install all.
if a reinstall isnt an option; try this:
start/run/msconfig.
Go to startup tab, and check to see whats loading at startup. Check what loads at startup. If it looks odd, check it out a tasklist.org (not 100% accurate, but a good start)
If something shows up as bad, uncheck it from loading at startup.
Do the same for task manager, except end-task non-needed processes.
If alot of things in msconfig/taskmanager are showing up as trojan/spyware/etc, a reformat/reinstall is probably best, especially if the pc hasnt been used much