It could be too many TSRs, the hard disk may be full, low ram etc.
I haven't heard of viruses that slow down mice, but I think cats make them move faster.
Computers are like sports cars; they drive fine in the beginning, but need tuning every once in a while or they start to puke.
You may have to suck up to; or threaten your son. Tell him you want your genes back.
The best way to work on your unit is 'hands on'. We can do what we can from the net, but your son would save lots of time and the cost of a local pro.
If you have potato chips and pop, you may be able to get a local highschool or computer club kid to 'tune it' for a good price.