How to download a virus guard to delete some viruses without letting the virus block it from installing???

Mylaptop is now infected by Trojans,blaster worms,Viruses etc...now i cant open or view most of the applications in my computer like msn messenger,skype,windows live movie maker,windows live media player etc...and also Doents files,presentations etc...it has been inside my laptop for almost 2 and a half weeks and im struggling with it...and also i dont have a virus guard on my computer..WHEN EVER I TRY TO DOWNLOAD AN ANTI-VIRUS the virus inside my computer wont let me install it!So i need any help regarding how to download a virus guard while not letting the virus stop it from installing!!!!HELP!!!!THANKS ALOT