Dude
I've read the other Replies ... all perfectly logical and correct; however, I recently had a similar situation develop on my Win2K system. I also run Ad-Aware, SpyBot, Symantec AV and Client Firewall, all behind a firewall router. I still caught a prog attempting to "call home", which got more and more insistent that it actually "talk to home", which in this case was HP!! Yah!! HP includes software for it's 1012 series laser printers that includes a "toolbox" along with the print driver. When I installed the driver (Aug '04) the toolbox was integrated in the package (no choice). Since I never used the toolbox after once checking what it did (connected to HP for tips, updates, etc.) I deleted the systray icon and forgot about it. Then, a few weeks later my system seemed a bit boggy ... slow doing stuff, but Sandra showed the same benchmarks. Then, the system got slower yet just sitting at the desktop. I popped up TaskManager and started checking ... Aarrggggh! 99% CPU utilization while doing nothing. I croaked the offending prog after noting it's name and discovered it was a HP print driver package prog. I uninstalled the driver package and got the latest driver from HP ... since initial release HP has separated the basic print driver from the toolbox package. You may have a similar problem, so check your TaskManager Processes tab.
Separately, I've found that some of the MS updates for Win2k since SR4 bog down the boot-startup process, but once all updates are installed thru SP5 Q818043 that "problem" cleared up.
Good luck