This has been happening to my 3 PC's recently, one has WinXP Pro, while the other 2 have Win Vista Ultimate 32 bit and Win 7 Ultimate 64 bit.
I originally had Zone Alarm Pro version 9.1.008.000 and Nod32 v18.104.22.168 installed on them all. Everything was working fine until I decided to upgrade to the latest versions. ZAP v9.1.603.000 and Nod32 v22.214.171.124.
It turns out after much uninstalling and reinstalling that Nod32 is the culprit.
I'm currently waiting for a response from Eset