Microsoft has released a very useful tool for Windows XP admins, the Change Analysis Diagnostic for Windows XP. Read about it here and download it here. When you install the downloaded package it puts the program in the pchealthhelpctrbinaries sub directory of your Windows OS directory (e.g., C:WINDOWSpchealthhelpctrbinaries).
You can run it from the “Start-Run” dialog, by creating a shortcut to statechangediag.exe or, from a command prompt. Basically it reads the system restore points information and presents it in a nice XML page. This will be very helpful in determining what new or updated program may be at fault when Windows XP starts behaving badly.