I ran across the same problem when installing Windows XP Professional on a Pentium III 933Mhz machine that I was working on.  I tried unsuccessfully for days to get the operating system to install, but it would stop at about 80% with the same errors you mentioned.  (I had 768 MB of ram in the machine at the time.)  I eventually figured out that one of the sticks of ram was bad.  I removed that stick of ram and Windows installed perfectly and has been running fine since.  
You can download the memory diagnostic test from Microsoft at
http://oca.microsoft.com/en/windiag.asp  Be sure to read the instructions.
P.S.  You may also want to consider increasing your ram to 512k.  I believe you would be much happier with the performance of your machine.  I personally find that Windows XP runs a bit sluggish on 256 MB, especially once you install all of your other software and have multiple things running.
Good Luck!
                
 T. Michel
                
 May 2005