Note: its best to use all three of these together for best protection.

Adaware » Homewww.lavasoftusa.com
Current version: Adaware SE 1.06 (Dont forget to update definitions regularly)
Description: Allows you to scan for and remove spyware.

Spybot S&D » Homewww.safer-networking.org/en/
Current version: 1.4 (Dont forget to update definitions regularly)
Description: Allows you to scan for and remove spyware.
Includes an imunise option to prevent some spyware infections (recheck after an update).
Also includes many other useful features in its advanced section.

Spyware Blaster » Homewww.javacoolsoftware.com/spywareblaster.html
Current version: 3.5.1 (Dont forget to update definitions regularly)
Description: Prevents a huge number spyware infections (enable new protection after an update).

Note: Dont use more than one antivirus program at the same time or conflicts could occur.

AVG » Homehttp://free.grisoft.com/freeweb.php/doc/2/
Description: A popular free anti-virus solution. (Dont forget to update definitions regularly).

Symantec security check » Homehttp://security.symantec.com/default.asp
Description: Online anti-virus and security scanner.

Trend Micro » Homehttp://housecall.trendmicro.com
Description: Online anti-virus scanner.

Panda » Homewww.pandasoftware.com/activescan/com/activescan_principal.htm
Description: Online anti-virus scanner.

Note: Dont use more than one firewall at the same time or conflicts could occur. This rule includes XP's firewall but does not include hardware firewalls like those included in routers.

Zonealarm » Homewww.zonelabs.com/store/content/catalog/products/sku_list_za.jsp?dc=12bms&ctry=US&lang=en&lid=nav_za
Description: Popular, easy to use personal firewall. (Dont forget to update regularly).

A² » Homewww.emsisoft.com/en/
Current version: 2.1 (Dont forget to update definitions regularly)
Description: Good lucking, easy to use anti-trojan scanner. (Dont forget to update definitions regularly).

CWShredder » Homewww.intermute.com/spysubtract/cwshredder_download.html
Current version: 2.19
Description: Simple tool for removing cws trojans.

Windows Security » Homewww.windowsecurity.com/trojanscan/
Description: Online anti-trojan scanner.