Ok, JOB #1: and this is without regards to any problem you may be having at
this time - you said the files are crucial to your work.  BACK UP those files
routinely - preferably to another drive.  Last thing you want are crucial
files of any kind only in one location and then have that one location become
unavailable such as through a hard drive crash or power glitch that hoses
even 'just' a single file.  To err(or) at a critical moment is typical, to
have a recent backup is simply divine!  

Scary, but true testimonials:  I've seen this happen myself (to me): worked
on bunch of stuff all morning, took a short (less than 5 minute) break -
drive was dead!  Only having a backup from previous day saved me from an
absolute disaster.  More recently a CPA had critical files relating to
several jobs on his laptop.  Even though the system had given him warnings
that things weren't right he continued to use it without backups of those
files.  It failed.  I spent 13 hours of my time and about $100 of his money,
recovering those files for him as a favor - later he gave me (and I accepted)
another $200 in appreciation.  So besides a near heart attack, his lack of
backup cost him $300 in cold hard cash.

As for your immediate problem: you might try a repair install of Office.  If
you have 2007, try using the diagnostic tools available to see if it notices
any problems with itself.

Try running chkdsk to see if the system can detect errors on your drive:  
Start | Run
type in chkdsk and press [Enter] and it will open a command window and begin
running the test against the C: drive.  
More information on the options available with chkdsk
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/c
hkdsk.mspx?mfr=true