You may wish to try the Panda ActiveScan Free Online Scanner.
Just click on the "Scan your PC" box.
http://www.pandasoftware.com/activescan/com/activescan_principal.htm

Download Ad-Aware 6.0 and scan your PC for spyware:
http://download.com.com/3000-2144-10045910.html?part=69274&subj=dlpage&tag=button

Symantec Security Check
http://security.symantec.com/sscv6/default.asp?langid=ie&venid=sym

3 Steps to Help Insure Your PC is Protected
http://www.microsoft.com/security/protect/

Frequently Asked Questions About Antivirus Software
http://www.microsoft.com/security/protect/antivirus.asp

Special note if you use AOL:

America Online installs its own connection settings that override
the ones that come with Windows XP.  America Online's
connection settings don't include a way to turn on Windows XP's
built-in firewall.

Visit the following web site for instructions on downloading
a FREE firewall program for your computer.

Ref:  http://www.updatexp.com/free.html

To secure your computer and prevent possible future security breeches,
consider installing a first-rate, comprehensive, internet security program:

Norton Internet Security 2004
http://www.symantec.com/sabu/nis/nis_pe/

-- Includes Norton AntiVirus 2004
-- Includes Norton Personal Firewall
-- Includes prevention of annoying web pop-ups
-- Includes Parental Controls
-- All in one, easy-to-install package