Sadly, I have seen this happen to all 14 of the Dell Vostro 200 computers I have sold; usually after a few days and/or restarts. Support for the problem is worthless, as everything revolves around reformatting and restoring.
Here is the only solution I have found thus far that keeps data and takes the least amount of time:
Using Windows XP disc, start from CD. At the first propt to install or repair, choose install. When Windows XP setup detects a previous version of Windows, choose repair. This process will complete an in-place upgrade (reinstallion) of Windows XP. Once complete, all of your programs will still be intact and unchanged, but you will need to reinstall all Windows updates, including IE7.
Good luck… because unfortunately I just received another call for one of these systems and I have to reinstall Windows tomorrow. Thanks for nothing, Dell.