Problems after Disk Defragmentation

> Hello. My computer has been running a little slowly for
> a few months now, so on Friday I ran my monthly Disk
> Defrag. procedure. Afterwards, I noticed my computer
> behaving strangely. Since I didn't create a restore point
> on Friday, I used the most recent restore point to see if
> this could fix this "new" problem...it didn't.
> My "new" problem revolves around the Windows Explorer
> program. Please note: A) My computer now still runs
> relaively slowly (at start-up and when loading programs
> into memory); B) My Windows Explorer program will
> temporarily shut down and then reset itself whenever I
> access files/programs using Windows Explorer; and C) I
> get this error message often when accessing Windows
> Explorer: "Windows Explorer has encountered a problem and
> will have to shut down. We apologise for the
> inconvenience."
> Can someone suggest a course of action or direct me to a
> newsgroup that could? I am an novice-to-intermediate
> level computer user and would appreciate any assistance.
> Thank you in advance.