you can probably use the shroud and replace the switch itself with a momentary. they make 'em in that configuration
edit:
http://www.connex-electronics.com/?u..._bat_slow.html
there's some here, google for momentary bat handle switch for more stuff to look through. you'll probably want to buy a separate shroud instead of buying a shrouded switch. also, i'm not quite sure what specs are necessary on the power switch for a PC, would be a decent idea to make sure it's the right amps/volts etc :p