My computer has been getting slower each day this week. Why is this
and what can I do to stop the crawl?

There are several reasons why your computer may be slowing down; a
lack of memory, something is utilizing your processor, or the PC may
be overheating.

I would first check to see that all of the fans inside the case are
spinning.  Also clean out any dust that has collected (while the
computer is off and unplugged).  If heat does not seem to be the issue
follow the steps below to help free up your system's resources.

http://answers.google.com/answers/threadview?id=745977
I am going to assume that you are running Windows XP.  If this is not
true please let me know.

1. Go through your Add/Remove program list and remove any application
that you don't use.

Start > Settings* > Control Panel > Add/Remove Programs

*if using classic mode

2.  Stop unnecessary programs from starting when Windows starts.

Go to Start > Run and type "msconfig" and click "OK".  You will get
the "system configuration utility".

Use this page for instructions on how to narrow down which ones to stop.
http://netsquirrel.com/msconfig/

3.  Make sure that you are running a good antivirus / anti-spyware program.

There are plenty of free programs out there for spyware, but I suggest
purchasing a program that does both from one of the major companies:

http://www.symantec.com/index.htm
http://www.mcafee.com/us/

Obviously there are many others as well.  The important thing is to
get one and keep it up to date.  Run a full scan of your system right
away.

4.  After all of this you should be running much faster than before. 
Lastly defragment the hard drive.

http://helpdesk.its.uiowa.edu/windows/instructions/defrag.htm

If you have trouble defragmenting start here:
http://forums.pcworld.co.nz/archive/index.php/t-27143.html

These steps will free up hard drive space, memory usage, and processor
usage.  If you have any questions about any of these steps please let
me know.

Thanks!

Lots of good advice