My computer is infected help please!?

Its a Dell Dimension 2400 with Windows XP Professional, its got a really annoying virus that wont let me download anything, I cant access the web (I dont know how I got through this time) whenever I boot up to safe mode my mouse and keyboard freeze, my Malware Bytes cant run or update same thing for my Security Essentials my system restore doesnt work and I dont have the reinstallation CD's can someone help me please!