If you bought the boxed version of the program, it's as simple as reinstalling from the discs. But if you installed from a download and didn't save a copy of the download, it's a bit more difficult. Adobe does not keep installation files for older versions of the program on its site.
Your simplest option might be to borrow somebody's installation discs (using the serial number Adobe provided when you bought the program) or buy a used copy on eBay.
Steve - whilst appreciating that Adobe does not keep the installation files for older versions online - what about the updates that usually appear a month or so after general release and are installed from within the program (i.e. are not a separate download that one can archive).
Are such updates still available once one has managed to re-install the original release?