I've been taken over

I'm running win xp. I believe I got the virus when my brother was trying use a usb wireless connection. He tried linking it to my dads and he got connected but the signal wasn't strong and he still couldn't get on the internet. Apparrently he left it on becuz when I got home there were a zillion windows and unfortunately I did the emergency shut down by pressing the power button and didn't bother reading what the window said. When it restarted, I tried to use word but it said application not found. I tried starting an executible file but it kept asking me what program I want to use to open it. I tried to run the task manager and it said it was turned off by the administrator. I tried to restore but it said it was shut off by administrator. I restarted my computer in safemode and logged on the administrator and can only get into a couple of things like administrative tools with limitations. I went into accounts and there is an unknown user that I can't get rid of. I'm at a loss as to what to do from here.