Plug and Play configuration error in Dell Dimension 4300S

I do not know the history of this machine except the client called me when it would not bootup.  My assessment was that it would boot, but could not find the HD.  I put an old drive into this machine, and it booted OK.  Original HD was not recognized in another computer so it appeared to have failed totally.  Dell supplied me a replacement 80 GB drive for the old 20GB.  The jumper is cable select, and at bootup the drive is recognized.  I then used the Restore CD (newly supplied from Dell) to install Win XP with SP 2.  Now at bootup there is this PandP error message.  I have replaced the MB battery - but get a message battery voltage is low!  RAM seems to be recognized, but I noticed it is not original - and the two modules are from different suppliers.  Other than the HD I have not replaced any hardware.  BIOS shows IRQ5 reserved; all others, available.   After pressing F1 to continue, Win seems to be installing.  Might mismatched RAM be the source of this problem?  What else do I look for?