The ballasts put out a tremendous amount of RFI (Radio Frequency Interference) even thought by FCC regs they are not supposed to (The FCC no longer has the resources to combat it)
Move the equipment further apart (not always possible)
Apply grounded shielding (not always easy)
Stop the RFI from radiating (easy but involved)
The easiest way is to apply Ferrite Chokes around the cables leading into and out of the ballast. Also do the same thing with the cordless mouse cords (and other's going into and out of the PC
You can get snap-on chokes at Radio Crap (sorry Radio Shack) but they are quite small and only work for thin gauge wires. Better bets are to search on eBay for Ferrite, that should pop-up a bunch of items.
Ferrite type 43 is good for 0 to about 30MHz. If your cordless mouse is in the 27MHz range, as most are, those would work the best. Type 77 works at higher frequencies.