GRRR AT COMPUTERS

Okay! Check it! I am fed up with my XP and Dell. People go ranting and raving at how good it was, and I have one and its nothing but a piece of _________! Yeah you know what I mean. While in the middle of an assignment a booter got onto my Yahoo Messenger and the only way to stop this was to unplug my tower from the wall. Well, this has happened before and it worked out fine... but this time when I go into the log in screen there is no box to where I push to get onto my desktop. So when you push ctrl alt del it takes me to the log in screen asking for username and password. Of course I just got this computer so I havent changed the account information yet... thus its under the person I got it from. I DONT KNOW HER PASSWORD though! Is there anyway that I can get onto my desktop yet... or is a lost cause and do I have to reformat/ wipe out my computer and install everything again. COMPUTER BRAINS, if you know, post a comment! I think I tried everything I could....