Reasons could be several, better if you throw in a few symptoms and some things you have tried to do yourself to fix problems.
There has been mention of third party Toolbars causing strange problems lately. Things like Yahoo, Google, Windows Desktop Search etc.
For IE I often suggest this step early on in the piece.
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.