64 bit system needs virus protection. Help !!

I just bought a new computer for my home business. It is a DELL 64 bit system and I am trying to find some virus program tht will work with it.
Will the Avast work? Can it be downloaded so I will get protection right away.
I am stranded on the web with no virus protection.
Please help. Here is what it says under properties:
System:
Microsoft Windows XP Professional x64 Edition
Version 2003
Service pack 1
It is DELL PWS 690 Intel Xeon CPU
5140 @ 2.33 GHz Then it hs 2.33 GHz again and then it has 4.oo GB of Ram
I paid to much for this computer to be out here where it can get viruses. I never dreamed it woud be so hard to find a program to support this system. I may have to send it back.
Thanks,
Raven2007

Yes, avast is compatible with this Windows 8)
Welcome to forums 8)