Windows updates not genuine. Hw do i resolve it?

I work with windows xp currently.Recently i had a msg dat updates r ready for my computer n i,without testing their genuinity, downloaded the update.But that was not a genuine one 4m microsoft,i learnt it nw.I gt a msg always blinking on my screen nw that i may b a victim of software counterfeiting.I tried 2 resolve the problem but it tld abt somr software 2 test the genuinity n that costs a lot.Is there any other method with which i cud solve this problem without spending a lot of money? Do i need 2 4mat the whole system then??? I dnt hav any antivirus prg installed n there may b some viruses in my computer 2.Wat shld i do nw?