Caoman,
There is something else going on here. Also we need a better explanation of what you are seeing. If IIS is "crashing" (you still have not defined this), it should have no effect on the operation of the OS itself, to include Terminal Services (unless you are using the TSAC client). If IIS is "crashing", then you should be generating a Drwtsn32.log entry and a User.dmp file. The suggestion to check your event logs when the system comes back up is a valid one. We need more information. Also, what has changed in your environment recently? How long was IIS up previous to these recent crashes and what changed? Have you added any new device drivers? any new software? etc? Are you running AV on the server? What happens if you disable AV and reboot the server? Does IIS stay up then?
Any other 3rd party services that may be running?
Also how long does IIS stay up after you reboot? an hour? a day? a week? If IIS is in fact Watsoning, then your best bet (unless you are familiar with debugging) is to get the User.dmp and the Drwtsn32 log to MS for analysis.
hth
Vic Rozumny, MCSE
Microsoft Corporation