Seriously, though...it's hard to say what the cause might be given the limited amount of information here; however, I can give you some general troubleshooting tips.
If the W3SVC service (or inetinfo.exe, its parent process) is crashing, then look to see if it is producing a Dr. Watson. Search for a file called drwtsn32.log and see if you can locate crash information for inetinfo.exe. If so, you may be able to decipher the crash dump and stack trace to determine why it is crashing. If you aren't getting a Dr. Watson and the service just terminates, you might try enabling auditing to see if some user or system process is doing something within the security subsystem that may be compromising the service.
Also, have a look at Microsoft Support Online (http://support.microsoft.com) and TechNet (http://www.microsoft.com/technet). You may be able to find one or more articles or technical documents that discuss your situation. Failing that, you should consider contacting Microsoft Product Support Services (contact numbers can be found at the Support Online site). Chances are they will be able to diagnose and resolve the problem very quickly.
Service crashes aren't fun, but in my experience there is almost always a way to resolve them. Note that changing the service's startup parameters won't help. That only dictates the service's behavior on system startup.
Hope this helps.
This was first published in January 2002