If you do research on this error such as http://www.google.com/search?&q=Iexplore+has+caused+an+error+in+USER.EXE you may find the common feedback is to eject baddies, Spyware or in a wide description "Parasites."
There is no single cause or cure for this non-OS issue, but I can offer what I would do.
1. Eject all parasites. I made a list at
CWSHREDDER moved to http://www.spywareinfo.com/~merijn/cwschronicles.html
2. Consider a little Windows Optimizing. A thorough article(s) are over at http://www.aumha.org/articles.htm
3. If this is just Windows running out of resources, then there is no fix or cure, but items 1 and 2 do help most.
I keep getting this message more & more frequently when using the internet. I'm using Windows ME & I also recently have had the following message resulting in my PC shutting down:
Iexplore has caused an error in USER.EXE (sometimes KERNEL)
Iexplore will now close.
If you continue to experience problems try restarting your computer.