This seems very suspicious. Your post didn't tell of what antivirus is on the machine, what you do to keep pests off the machine (spyware) and if you let Outlook Express preview email or much else.
Try the usual scans with Adaware, Spybot, Cwshredder and Housecall, then look up an duse IEFIX.
I recently repaired my XP home addition. Now everytime i set up my outlook express to receive and send my web mail it works but when i shut down and start my machine the next day all the previous days work is missing and it boots as if it is the first time I am logging on to the machine.
The folders i had created on my desk top end up in the recycle bin.
I cannot seem to use ms messenger as it asks me to register my net passport.