I really do hope you fix it, it gave me a hard time when I had no idea what it was or even what to do. You can start your pc on save mode by pressing F"number", it is not always the same. It might be F12, F10, F8 or F2, or look at your screen when you start your computer.
Keep pressing one of them and you should be seeing options after starting your pc, chose safe mode with networking...press yes. Then download antivir and a saaser worm scanner. And use this too : http://vil.nai.com/vil/stinger/ b4 uninstalling or installing anything.
If it's not a worm, then it's got to have something to do with that crappy Nvidia chipset. For two years I've had NOTHING but major problems using Nforce 4 based boards. Most of them had to do with the Ethernet card and/or firewall setup, and some the IDE setup. The most common were the Ethernet problems, and sometimes it involved random reboots when using the Internet (heavily). I switched to an Intel chipset while waiting for my Core 2 chip, and I'm happy to report very few problems while using a temporary 805, unless I overclock it to much.
I would suggest disabling the Automatic system restart and checking if there is a BSOD. If there is - look for an error code/description or a file name.
Although this is probably irrelevant ... I have experienced the same problem with modems (dial up) conflicting with HT (Could be the same with Dual Core). Whenever I was online after 4-5 mins I would get a BSOD and IRQ_LESS_OR_EQUEL (or something like that).
Check for the BSOD.
Also if you could tell us what kind of connection you have - and how you connect (Modem / router e.t.c) it could really help.