Gabriel:
If your system is already infected with a computer virus, it may prevent an anti-virus program from properly installing. Your best option, then, may be to download and run a virus removal tool made specifically for that virus.
Is your system infected now? If so, what is the name of the virus, Trojan Horse, or Worm? If you give us the name we may give you the link to a site where the removal tool may be downloaded.
Doc