Lots of suggestions, but products I have no experience with. Might I suggest a packet sniffer. Many free versions out there that work well. They look complicated, but aren't. They simply catch packets (TCP/IP) over the line - cable I am guessing and will give you an IP of those trying to gain access. Most will be between your modem and the ISP, but soooo many will be the hackers using automated probe software to see if there is a way in; you can test your PC using tools found here

http://www.grc.com/intro.htm

He is a sharp guy and suggests some good tools. Root kits can be "stuffed" into other files - jpegs for example without changing the size or date of the host file. It's a common hacker trick I learned. When that's done, no software will find them. I think it's called slip streaming or something like that.

Sniffers are found here:

http://netsecurity.about.com/cs/hackertools/a/aafreepacsniff.htm

A software firewall is here. Free and I have used it:

http://www.zonealarm.com/security/en/computer-security.htm?cid=W1000281

I am 99% sure something on your machine is logging keystrokes or traping packets, sending them (copies) to a hacker for cracking (packets) or use (key strokes).

If the common thing on the hacks is the PC, I would guess it is still on the PC. Though things may see to be back to normal for rnow, until I hunted down the root cause of the hack, I would be at all confident.

I used to know a great deal about hacking, but it is an area that moves pretty fast and I haven't kept up. I hacked my own network and people who gave me permission to do penetration testing on theirs, but never black hat stuff. I was strictly a white hat - or good guy.

Best of Luck. I despise hackers.