Recently my computer had gotten infected with some viruses. I went out and bought Defender Pro 2005 but I've run into a few snags. Here are the problems as well as questions:

1-Despire being updated as soon as it was installed I'm still getting System Alerts and messages reading Critical System Error on my taskbar. All new scans bring up nothing.
2-System Alert bubbles read like this:
System Alert: Trojan-Spy.Win32@mx
Type: Spyware/Trojan
Vulnerable: Windows 95/95/ME/NT/2003/XP
Description: Spyware program that sends confidential information to a remote attacker
3-After installing Defender on my computer I attempted to install it to Brandy's/Asi's computer but afterwards I get an error message saying there was a problem and that the virus logs are corrupted. Attempting to update to new logs doesn't work and reinstalling doesn't work either. Solutions?
4-The main purpose of my computer is for a 2nd player to play World of Warcraft. Both computers are linked to a broadband router. Will leaving my computer connected to the internet potentially infect Brandy's computer? If not then are there any programs I should directly avoid using so that I don't infect her computer?

Brandy had suggested this because it was cheap at Target. Only $9.99 yet it doesn't seem to be doing much. I really can't afford Norton or McAfee at $39.99-$49.99 at the moment but I really need to get this computer cleaned up as well as the other one protected. Is this a case of getting what I paid for? Am I doing something wrong that's causing this computer to not be fully cleaned despite the virus log being as recent as 8 AM today? Please help. I'm not tech savy and I really need this taken care of.