The WORST thing you can do, when you suspect your Windows computer is infected, is to download free software off the internet. (Non-free is not much better.) 99.99% of what you find will leave you worse off than before.
But there are exceptions to the rule. The exceptions we know about are listed below. Our endorsement of these products is unsolicited and uncompensated, based on our personal experience. If they work for you, make a donation or purchase the full version from the provider.
These links are provided as a public service in no particular order.
A very good tool for detecting spyware, trojans, and other potentially unwanted programs that are often missed by even the best antivirus programs.
Spybot Search and Destroy
Beware of the bogus version of this software. It has such a good reputation that it is often imitated. It has a feature that immunizes your web browser from visiting many bogus sites and detects problems that antivirus programs fail to detect. Has a "teatimer" option that looks for changes to you system, and gives you a chance to roll them back. Always download directly from the "real" site.
Emergency System Recovery Disk
If your Windows or Linux system has been rendered non-bootable by malware, restore it for FREE with Dr.Web LiveCD.
AVG Free provides you with basic antivirus and antispyware protection for Windows and is available to download for free.
AVG Free Home Page