Wish I could be responding with the solution but I have a desktop that has shown the same symptom for ages and I cannot find any cause. Computer freezes and then carries on as normal after a short time, 20 secs being typical. Sometimes on power up the first screen stick as though it freezes at that point before BIOS even completes. PC is a Philips Freeline running XP Home SP2.
I am convinced it is hardware related as the effect is like some component holding onto the CPU cycles for a period and then continuing. No data is ever lost, on video playback things carry on, downloads carry on through a freeze. All very strange. Had to do a rebuild of XP recently and the problem is still there so not convinced that the "restore" answer you may get will resolve it. Of course the cause of your problem may be entirely different to mine.
I posted on here middle of last year but no one responded so I will be interested in any you get.