Try Avira antivir
It's free and it's pretty good overall
http://www.free-av.com/en/index.html
Reasons why you can't install one can be
-You have an antivirus that is blocking installation of other antivirus software (unlightly)
-You are already infected, your computer runs weirdly and refuses to install an antivirus
So, do you have weird issues ? popups, that red little windows security center notification in the bottom right poping, errors, freezing....
What you can try is, reboot your computer,
Press F8 once every second before the windows XP logo (if you see the xp logo, you are too late)
Pressing F8 will make Windows Ask you how you want to start
Start Windows XP With Networking.
If you are connected to a router, you should have internet with that option.
Add remove software, remove whatever antivirus you have (Specially if it's symantec or norton internet security)
Go to the link above, download Avira Antivir and update
Go to google, search malwarebyte, download, install and update.
Scan your pc once with avira, scan it then with malwarebytes.
Should clean the pc if it has weird issues that is.