If you can run batch files, then you can make one with the contents "start cmd" to start up the command prompt. Assuming you're running Windows XP or below, then you can do a lot with that provided the permissions don't muddy it up. To actually edit anything in Windows Vista or higher you need administrator permissions though, so yeah... Oh, a neat trick for bypassing website bans is by going to the websites IP instead of the URL. For example, instead of mpgh.net you could go to the command console and type "ping mpgh.net" and you'll see that the IP for MPGH is 188.8.131.52. You can just go to that and it should bypass any DNS blocks your school's placed.
Sidenote: Oh look, you can't even access MPGH via it's IP. That's just an issue with VBulletin so it's a bad example. Other sites like Facebook should work. But yeah, if you have open access to the command prompt you can do pretty much anything. If you have any function in mind particularly, just Google it.