Problems when using macro created in Excel 2000, in Excel 2003

I tried to run a macro originally created in Excel 2000, in Excel 2003 application. It was running unusually slow and I stopped it by pressing Ctrl+Break. The macro stopped running, but now every time I try to run it again, it gives a pop-up message 'the code was interrupted'. You can press Continue, End or Debug. If you press continue, the message pops up again in a second and you have to press it over and over. The macro works fine on other PC's, but I am having problems now running it in both Excel 2000 and Excel 2003. (Tried rebooting the PC as well...)

Any ideas?