I only just started having this EXACT same issue. Our IT couldn't even figure it out, and then by accident I think I found out the cause.
Apparently it seems like Google Chrome is conflicting with Microsoft Excel 2003. It's only recent too; but if I have Google Chrome running it won't copy and past formulas and won't even allow me to right click a cell and choose the way I want to paste, normally.
BUT as soon as I shut down Chrome - BAM! It works normally again. So if you're also running chrome it may be the cause of your problems too. If not, I would try running XL with no other programs running and then see what happens. If it works I would then start running adiditonal programs you usually have running in the background to find out what triggers your issue.
Good luck!
ETA: I was just going to post the issue on the Google message board and did a quick search for "Excel" and found a bunch of peiople having the same issue. A Google tech replied and said that yif you have Skype "Click to call" installed it's what's causing the issue, so you need to uninstall it and then everything goes back to normal and you can run Chrome and Excel at the same time.
