IBM Model M 84-Key

I recently found one of these beauties at my local thrift store for $3.99.  It is in near perfect condition.  I think it was previously used in an office , because when I bought it there were a bunch of pen marks on it.  It is an IBM Model M 84-Key Part no. 1370475 with a PS/2 port.  I connected it to my PC and it seems the "numpad" keys (7,8,9,U,I,O,J,K,L,<,>,?) are stuck in numpad mode.  I can't get it to type the letters.  The only workaround I found to this problem is to boot up my PC with it connected, then unplug the cable that plugs into the keyboard.  It seems to fix it until the PC is shutdown or rebooted.  Does anybody know enough about these keyboards that I can fix it permanently?  If you are curious as to what my keyboard looks like, this is the one pictured:

http://www.clickykeyboard.com/_ebay/2040131/2040131-001.jpg

Try going into your BIOS settings. You should have an option that dictates whether NumLock is on or off at boot.