Why aren't computers like cars?

So here's the problem, our 2 yr Norton Anti-virus subscription expired last week so I thought it was a good idea to renew it. However, at £85 for another 2 yrs I thought this was a bit expensive so I purchased a different anti-virus program much cheaper to replace it. I then went about trying to protect our computer by un-installing the Norton to download AVG. Simple I thought, but oh how wrong was I?!?, turns out that by not un-installing it properly I now can't get any internet connection - not good!!. As a result, I've left the problem for Claire to sort!. Frustrated by the computer, I turned my attentions to our Golf instead and more importantly the replacement of the front discs. So given a couple of spare hours on Saturday evening the car went up on the ramps and I started to take the wheels off and then the brake calipers off to then replace the discs themselves. Now why are computers like cars?? I can take off (un-install) the old discs, I have my new discs along with the new bolts and new sleeves to go on (bit like the new software) and I can see if I go wrong then sort it, but with a computer - now that's a different matter. Grrrr!!