ive bought a new pc for my dad an oap will he be able to work it
Linux does not require:
anti-virus software as it does not get infected.
defragging.
If runs faster even on old equipment.
Generally, Linux is not more difficult to install than Windows and offers a similar amount of eye candy. The difficulties lie in other areas, though.
If he is up for general office work (word processing, spreadsheets, presentations) and little games (solitaire, mahjong) then Linux has everything he needs. In those areas both operating systems perform equally well. There are also programs for many scientific purposes, editing graphics and desktop publishing.
Take a look at Mandriva One to see what a basic Linux distribution contains and how it compares to Windows.
Once the machine is set up (nice big icons on the desktop for "Internet", Word Processing, Spreadsheet etc) it will probably be as good or better than a windows machine. However, the set up is often the biggest headache.
The main things, monitor / keyboard / mouse / sound are usually well detected and work instantly. Biggest headaches tend to be: 1) Getting the internet to work - especially if this involves wireless, 2) Getting printers and scanners to work - scanners are often more problematic than printers, 3) Getting things like hibernation to work - not too big a hassle, as startup time tends to be better on linux than windows 4) Getting the best performance out of your graphics card (but then, I'm assuming your dad isn't a big Half-Life fan?)
If you already have linux installed, set up and working, then the learning curve would be no more than that of learning to use windows. Not to mention the likelyhood of getting a virus gets cut from about 1 in 5 to about 1 in 50000. I've been using various flavours of linux for about 7 years and have never had a virus. Ubuntu is great at keeping itself up to date.
Things you might have to watch for include: Ubuntu trying to keep itself up to date and breaking something else (very rare), your dad wanting to install windows programs (wine is getting much better, but still only works with about 40% of programs) and bugs in the software (not seen too many of those... apparently windows users know a lot more!)
As for choice of distribution, choose something like Ubuntu, which is easy to use and has good package management. Also, make sure that you put /home on a different partition or disk, as this makes reinstallations of linux a breeze in the unlikely event that you need to!! :D