this can happen. WinXP is good at recovering from 'serious errors' although they aren't always as serious as it may seem.
Often, a restart like this can occur when something - usually an application - tries to write to the Windows protected memory area. Windows will go to any lengths to guard against this, and will sometimes simply dump everything and restart.
Unless it happens on a regular basis I wouldn't worry unduly - by all means let WinXP report the error. What happens is that the error log is sent to Microsoft, and a database search is automatically triggered. If there's a fix for the problem you'll be presented with a web page containing it - otherwise (and more often than not) you'll simply see a page telling you that there's nothing helpful Microsoft can tell you.
FE has covered possible causes for the restart, but I thought it may be of interest to know that there is an MS update for the error message:
"System Recovered Error Message Update Download size: 2.8 MB, 6 minutes This update addresses the "'System has recovered from a serious error' message appears after every reboot" issue in Windows XP, and is discussed in Microsoft Knowledge Base (KB) Article Q317277. Download now to prevent the error message "System has recovered from a serious error" from appearing each time your computer starts. Read more..."
(Then again there's a lot I don't know). But what I mean is that occasionally I get an error message from XP and asked if I wish to send a report to Microsoft. I always click to send the report but I've never been presented with a page from Microsoft outlining the possible cause of the problem - or not. Is there something wrong with my system?