I keep getting a windows registry popup that tells me i have a critical system error and it keeps telling me to go to different websites to download a registry cleaner. I purchased a registry cleaner from a local store and it still didnt work. Plz help

You are infected.

If you want to empower yourself & have a go, here is all the info needed to get your comp clean, no need to buy anything, stick to the free programs, just a matter of finding sites that suit your skill level & using a search engine like Google to research each problem file found.
Once clean, you need to research on preventitive measures to stop yourself getting infected again.

Securing Your Computer: Temporarily Disable Real Time Monitoring Programs.
http://wiki.castlecops.com/Securing...
If Your PC is Infested w/ Spyware & Adware...
http://spywarewarrior.com/sww-help.htm
Trustworthy Anti-Spyware Products ( 3/4 of the way down the page )
http://www.spywarewarrior.com/rogue...
http://pandaman.my.contact.bg/Gen_M...
Note !!! Now, if you wasn’t able to clean your computer, perform the following, Special Clean instructions.
http://pandaman.my.contact.bg/speci...
http://www.malwarehelp.org/how-to-c...
http://spywarewarrior.com/sww-help.htm
http://spywarewarrior.com/viewtopic...
http://spywarewarrior.com/viewtopic...
http://spywarewarrior.com/viewtopic...
http://spywarewarrior.com/viewforum...
http://www3.telus.net/dandemar/slow...
http://discussions.virtualdr.com/sh...
http://www.wilderssecurity.com/show...
http://forums.techguy.org/security/...
Use HiJackThis to track down or check for possible infections. Read this link 1st, it has step by step.
http://www.wilderssecurity.com/show...
Important: Create a specific folder on your hard drive called HijackThis to keep its backups.
You can do this by going to My Computer (Windows key+e) then double click on C: then right click and select New then Folder and name it HijackThis. Download and unzip HijackThis.exe into this folder.
http://www.merijn.org/downloads.html Or, http://tomcoyote.com/hjt/
If possible run HJT in Normal mode ( not Safe ) with all your normal startup's working.
HijackThis Tutorial - How to Analyse your own log.
http://spywarewarrior.com/viewtopic...
http://hometown.aol.co.uk/jrmc137/h...
http://www.bleepingcomputer.com/tut...
http://www.malwarehelp.org/understa...
HijackThis log file analysis ( online )
http://hijackthis.de/index.php?lang...
Or,
http://startup.networktechs.com/pag...
http://hjt.iamnotageek.com

You may want to google for RegscrubXP. It's free and will clean the registry junk out that is no longer used on your PC
http://www.majorgeeks.com/download....

Hopefully my advice will help you...Please post back with your results....thanks