Quote: "Just want to make sure because if yes then i will install anti virus."
Either a) you pissed off a hacker, who proceeded to threaten you, or b) you're using utorrent/limewire and just had the shocking revelation that your IP address is free to see (you dirty pirate you).
But get anti-virus no matter what. You really can get them from safe-seeming sites. I recommend Avast! (! is part of the title). It's completely free, fast, and I've yet to get a virus.
Windows Vista Home Premium Intel Pentium Dual-Core 1.6 Ghz 1GB DDR2 RAM GeForce 8600GT Twin Turbo