|Your four choices to diagnose and test the desktop sounds like maybe 95? Your USB mouse won't work very well in there unless you have previously jumped the USB hurdle dual booting with 95.|
Perhaps we are talking about the Dell Partition diagnostics? I don't have those here, when I try to boot that partition I go instead to the same as normal boot up which is XP dual boot choice menu and mine is Win 98 which had better USB support than 95 so my USB mouse works OK there too. I am using a PS2 keyboard because I didn't want to loose Function keys operation if ever there were issues with USB in 98.. IF your mouse is PS2 with a loose connection, that is not good at all as there are many warnings to never connect a PS2 mouse while the system has power. How bad it is with a loose connection I really don't know enough about it to say.
Mine automatically does not go thru F12 choices, it goes to XP/98 choice screen and I select with arrow key which I want to boot. Enter key makes it happen. If you had a working floppy drive and disk you could boot that to Win 98 and run a non-destructive hard drive test program to see if there any issues with the hard drive.
Did you want to boot and use your old OS or start over? Do you have any of the support CDs that came with the machine? Dell Recovery CD for example, might be others too.
Barts Disktool can test your hard drive non-destructively from what ever source you can boot from.
Rufus will make a boot USB out of almost any USB pen drive. And you can boot USB pen drive or Linux distro CD to have a look around. Lots of choices. What date does your computer think it is? This would be very telling as to battery condition which you would be needing to change with or without the 3 year dirt nap anyway -- buy a battery already. Be done with it for $4.