NOTE TO ALL WINDOWS XP USERS...
This issue is likely to happen regardless of which browser you use to do you web browsing. I even found the problem on a PC where the user had (months ago) removed IE.
It doesn't seem to have any weighting if you are using IE6, IE7 or IE8... if you have IE on your PC, you will likely (at some point in time, unless Microsoft release a patch for their patch) come across this missing DLL issue. In every single case I have worked on so far, the user has had no interaction with installing an update for weeks. So I believe it to be a forced update of Microsoft's in an effort to ensure that ALL windows XP computers have IE8.
The fix is easy enough - whilst your PC is working (or if not, get access to a working PC), burn a copy of the iertutil.dll (normally found in the system32 folder) to a CD. I say CD because a flash drive is not going to be detected at this stage in the Windows boot procedure.
If/when you do have the error, press Ctrl+Alt+Del, and while on the "Applications" tab, go to File > New Task (Run...). Type in CMD and press enter (you may be bombarded with error messages while trying this - just dismiss them).
When the command prompt appears, put in your CD that you prepared earlier, type D: (or E: or whatever your ODD is) and press enter.
Type DIR and press enter to see the contents of the drive you are on. If you got the right one, you will see the iertutil.dll listed.
Type COPY IERTUTIL.DLL C:\WINDOWS\SYSTEM32 and press enter.
If the file copied successfully, type SHUTDOWN -R and press enter. Wait for the computer to restart and your work is done.
My suggestion to EVERY Windows XP user is to burn this dll to a CD just as a precautionary measure... it could save you hours of work and $$$ if you had someone else do it.
Thanks Microsoft!