well one of the main reason people downclock is to cool down the processor that they think is too hot ( 50-60 degrees is still okay friends)

antoher one is they need super silent machines so that they dont need super powerful fans and dont mind the reduce speed. websurfing, office and such would not see a performance increase even if you have a 2ghz processor downclocked to 1.5ghz processor. unless u play games...but the temprature difference is quite high and they dont want/need to invest in high end cooling.

actually ur friend can create antoher profile in winxp that does not have this applications( Games)

its easy, if u dont want the headache and prefer another way,
install it some folder and remove the icons( hide installation folders )
or use GPE to limit access to certain area's of ur disk

yes
and make sure u set password to the administrator account that you currently use.
if can, hide the folders that your account uses in windows
or then again use group policy editor to do necessary security policy's u want to enforce