Perhaps a visit here will get you going in the right direction.
Removing adware & spyware
How to beat your advertising popups & other browser nasties
FAQ608-3482
Removing adware & spyware
Before Posting a Hi-jack log file - Best Practices
FAQ608-4650
Make sure your options in IE/ Tools/ Internet Options/ Advanced are correct.
Uncheck the boxes for "Install on Demand" for IE and Other programs (two boxes).
Try running temporarily with the third box "Enable Third Party Browser Extensions" unchecked and see how this goes.
"This specifies that you want to disable features you installed for use with Internet Explorer that may have been created by companies other than Microsoft.
If you encounter problems with Internet Explorer that you cannot resolve, you can use this option to help determine if third-party features are causing the problems without uninstalling the feature. You must restart Internet Explorer after turning this option on or off."
If that stops your errors, you will know where the trouble is.
This article and the program BHOcop might be useful to you.
Some general things to try.
See if System Restore will get you back to a restore point before your problem with Explorer.
Try Safe Mode (Do you have this problem in Safe Mode?).
Try running ChkDsk to check your drive for errors. Right click your Drive icon/ Properties/ Tools/ Error Checking.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
How to Reinstall or Repair Internet Explorer and Outlook Express in Windows XP (Q318378)
You can reinstall IE in Windows XP by clicking Start, Run and entering the following command:
rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 c:\windows\inf\ie.inf
(where c: is your Windows drive).
You will need to have your XP CD available. If you re-install remember to update it.
Using SFC /Scannow at the command prompt will help in repairing IE.