Windows XP/SP2 wont install programs

[Conos]

I reformatted today, and then updated to SP2. When I did, firefox stopped working, so I tried to dl a new copy after uninstalling. When I tried to install firefox, windows first said
"The publisher could not be verified. Are you sure you want to run this software?"
(I downloaded it off of mozilla's website.)
Then when I made it run anyway, it failed and said "File is corrupt".

I had to update to sp2 because my sound drivers wouldnt work with sp1, now I cant install anything. Is there anything I can do about it?

 

[kimsland]

Run CCleaner
Then try Firefox Download again
The verify signature is ok just to say continue, it's actually in Right click My Computer, properties area (on of those tabs) to turn this off
Actually Hardware Tab the "Driver Signing"

 

[Conos]

When I try to download CCleaner, it gives me this error when I open the setup:

NSIS ERROR

Installer integrity check has failed. Common causes include incomplete download and damaged media. Contact the installer's author to obtain a new copy.

 

[kimsland]

Close all open applications, including Internet Browsers

Start-->Run-> Cleanmgr -> ok
Then press OK again

After MS Clean Manager finishes cleaning, then try downloading and running CCleaner again

If you have further issues, you can also try downloading the portable (no install) CCleaner from here: http://www.ccleaner.com/download/builds/downloading-portable