If you need the best quality you need a dedicated slide scanner. The recommended ones are currently the Minolta 5400 and Nikon coolscan, but they will cost £400-500. There are cheaper models around, but I've not seen any recommendations.
As a cheaper alternative, some of the newer flatbeds come with transparency adapters which will allow you to scan negatives and their quality is now getting quite close to film scanners. They also allow you to scan larger format negatives, if you have any. The Epson 4990 has had a good review here
http://www.photo-i.co.uk/Reviews/int.../Page%2013.htm.
It is a slow and tedious process. My old Minolta film scanner takes about 2 minutes to scan each slide or negative, and doesn't have dust removal software so you have to spend quite along time spotting the scan afterwards if you want a reasonable final result. The Nikon scanners and the newer Minoltas, as well as the better flatbeds come with automatic dust removal software which eases this problem, at the expense of longer scan times.
Bill