ok thank you.
now i understand, i think:
you are playing pr and while you are ingame on a server (map has already fully loaded) your computer just suddenly reboots
this has a wide range of possible causes
here are some probable causes:
this can be due to overheating (self protection)
or hardware failure (something is broken [e.g. capacitator leaking])
or a critical error (bsod)
i think its the latter, do this to make the bluescreen last long enough to read it
Disabling Blue Screen Of Death Auto-Reboot - Windows XP Tips and Tweaks on Tune XP
then write down the error code (0x000xxxxxx) and error message (e.g. PAGE_FAULT), google it, post it here
what are your system specs please (operating system, gfx card etc)?
do you see graphical artifacts (textures look strange, rainbow colors, no walls/ground, purple check patterns) right before the reboot or earlier?
something else to try in the mean time: update your gfx drivers