Computer will not stay in sleep mode

My computer wakes from sleep mode every 15 mins. The only way I can keep this from happening is to use zone alarm and stop all internet activity. It will still wake one time after I lock the internet, but the second time I hit the sleep button, it will stay in sleep mode until I go back online.

This problem started after I backdated my computer clock, about a year ago, (hoping to get information out of a program that had expired). Since then I have used the system restore disks and restored my computer to no avail. I've checked everything that I can think of that might be trying to access the internet, but can't find what it is. I would appreciate any help.



HP pavilion a730n, pentium 4- 3 GHz, XP Home SP2, 1.5 G DDR 400, 200 G SATA HD, on board video and sound, AVG free v.7.5.432, Zone Alarm v.6.5.737.000, DSL internet connection.