Why so many ICMP requests when nothing is running?

Even if I have no other applications running, I can go to the activity page of my firewall software (have tried ZoneAlarm, and am currently using Comodo) and watch as hundreds of ICMP attempts (blocked by the firewall, but attempted, nonetheless) go by.

My guess is that I have some sort of virus, trojan, worm, or spyware, but no matter which virus scanners or anti-spyware programs I use (AVG, BitDefender, TrendMicro, Kaspersky, AdAware, Spybot, HiJack This, A-Squared), the scans always come back saying that my system is clean.

Why would my system be sending out so many ICMPs? What else may I try to see if this is a virus type of thing?

Anything that would help unravel this mystery would be greatly appreciated.

Thanks!