You may have a rogue scareware AV that's trying tou coon you into paying to install a trojan malware. It's probably trying to prevent you from getting a real AV.
Try to restore your system to a previous restore point.
From another computer, download one of those antivirus and place it on a removable media:
Microsoft Security Essentials
Install it on your computer and do a full scan.
Also, get Spybot Search & Destroy.
Install it and do a full scan.