When IBM designed the original PC it was with the intention that it would be easy for any user to upgrade it, which is why the power supply is in a sealed unit, so that ther are no dangerous voltages inside.
To replace the drive, all you need to do is:
Remove the cover
disconnect the two cables on the back of the drive
remove the 4 screws holding it in and slide it out.
Then just reverse the operation to install the new one.
The whole job should take no more than 10 minutes.
If you are really too nervous, then just take it to a local PC shop where they will probably charge a few pounds (in addition to the cost of the drive).
Alternatively, you could get an external drive.but it will be more expensive, and won't offer such good performance.
Here are some external drives:
http://www.storagedepot.co.uk/Products.aspx?ac tion=list&type=54&gclid=CK7b-OD9hYoCFRA1Xgod_A nbOQ