The instructions given by Ferrick are pretty clear. Basically this is what he did.
1) Copied his current save game (can be found in C:\Program Files\Diablo II\saves). There will be a bunch of files in there. Just copy all of them into a new folder. You could even copy them to the desktop, so long as it isn't in the Diablo II folder.
2) Uninstall the game and delete all the leftover files in the C: drive. i.e. The original Diablo II folder. I'm pretty sure this doesn't need an explanation.
3) Run the check disk. Done by accessing My Computer. Right click on your C: drive and select Properties. Once the Properties window opens up, select the Tools tab. You should see something called Error-Checking. Click on Check Now and follow the instructions given.
4) Defragment your C: drive. Done by accessing the same tab in the Properties window. Just follow the instructions in (3) but this time select Defragment Now. Pretty self explanatory.
5) Once Windows has done its defragging duties, reinstall the game.
6) Then patch it to the latest patch. At time of typing it is the 1.12a patch that can be found
here.
7) Remember the copies of the saved games you made in step (1)? Copy them back into the place you took them out of in the first place. I.e C:\Program Files\Diablo II\saves.
If that isn't clear enough, or you fail to understand something, please post here and state your problem. We will do our best to help.