The dell dimension A07 bios does support legacy usb iirc. I think the problem might be windows, rather than anything else:
I've had this problem a couple of times on Dell boxes running windows XP.
1) install windows XP and everything is happy.
2) shut down the box, change usb keyboard/mouse, switch back on.
3) Computer refuses to see the keyboard and mouse. You cannot log in, you can't do squat.
This happened after I reinstalled windows on a customers machine and then delivered the tower back to his house. After 20min of faffing about I drove back home, got my mouse and returned to his house.
Booted the computer with the mouse used during installation and suddenly it worked. Clicked on the users name to log in as him (no password) and once the desktop loaded I was presented with a screen saying "Hey, you've added new hardware! Its a keyboard and Mouse! Click next to complete the installation and use the new hardware!".
Click next in the New Hardware Wizard and have the new keyboard/mouse work fine.
As Ward mentioned in the comments, You could clone the EFS data (using ghost or dd on a linux CD) and then not be worried about loosing it.
If you have a PS2 mouse/keyboard (or rather if the motherboard supports PS/2 then you'll be fine, except IIRC then it doesn't.