XP Activation Looping after activation

I have now seen 8 instances of the following problem: Windows xp Home installed on a fresh build, activated via web or phone. All updates done SP2 etc.. drivers updated and installed etc. Laptops from several man. Dell, HP, Gateway as well as custom builds are included. After the product activation, the computer runs for several months with no problems. On a given day the Product must be activated dialog pops up, the customer click on OK and another windows pops up "this product has already be activated" appears. Click OK and the log in pops up and the cycle repeats itself. The only solution seems to be a OS reinstall, and reactivation. We have had several case where the customer has had this happen 3 times. Customers are connected to internet with Dialup, Cable, and DSL seems to be no pattern. I cannot boot to Safe Mode either, just a command prompt. Resetting the wps.dbl and .bak files do not help. any one had anything like this? Bob