How do I get rid of adware and viruses in my old windows xp pro for free or is it trash?

Try some of these:

Spyware, Adware, Trojans, Malware, Dialers, Popups:

http://www.download.com/Ad-Aware-SE-Pers...

http://www.ewido.net/en/download/

http://www.javacoolsoftware.com/spywareb...

http://www.download.com/Bazooka-Adware-a...

http://www.javacoolsoftware.com/sgdownlo...

http://www.safer-networking.org/

http://www.download.com/HijackThis/3000-...

http://www.emsisoft.com/en/software/down...

Online Scanners:

http://www.webroot.com/consumer/products...

http://www3.ca.com/securityadvisor/virus...

http://www.pandasoftware.com/products/ac...

http://www.bitdefender.com/scan8/ie.html

http://www.windowsecurity.com/trojanscan...

http://www.kaspersky.com/virusscanner

http://www.spywareguide.com/onlinescan.p...

http://housecall65.trendmicro.com/

Spyware, Adware, Trojans, Malware, Dialers, Popups List:

http://www.softpedia.com/catList/104

http://www.download.com/Adware-Spyware-R...

http://www.majorgeeks.com/downloads31.ht...

http://www.filehippo.com/software/antisp...

Antivirus:

http://www.grisoft.com/

http://www.avast.com/

http://www.free-av.com/

Antivirus Lists:

http://www.softpedia.com/catList/1

http://www.download.com/Antivirus/3150-2...

http://www.majorgeeks.com/downloads29.ht...

http://www.filehippo.com/software/antivi...