I have a Windows computer on my network that starts extensive network communications and hard drive usage at the same time every day. There are no scheduled events or such things. It seems to me that it probably has a rootkit. What are some other symptoms I can look for regarding rootkits?

Since this is about a Windows computer, I have moved it to General.

Well check what IP it's talking to. If the machine has been comprimised then you mightn't be able to trust what it tells you but the router will keep accurate logs, find out who it's talking to. Once you know that it probably won't be too hard figuring out why they're talking. Go from there.

I'll do an ethereal thing today or check the router logs. I'll post after that.

I just ended up reformatting. It seems to be fine now.

Well naturally your computer is more secure with no operating system installed.

if the rootkit makers were smart, you wouldn't notice a thing (untill its to late).

But since you are talking about windows, it was most likely some script kiddie, or someone who just dont care if you find the rootkit (since there are so many other windows computers around, who cares if you find a rootkit?). In such cases, exessive network traffic would be a good sign, especially connections on a regular basis to the same computer outside your network, strange files/logs, strange processes running, etc. Other subtle tones might be "J0 g07 PWNED" all over your desktop, in big red letters.

"J0 g07 PWNED"...you crack me up, SciYro!