I've got a "C:\download\sonicfoundry\" directory on my hard drive with subdirectories for each different application. I store the download files, serial numbers, purchase receipts, and accessory files in each appropriate directory. After each purchase or update i'll burn the whole sonicfoundry directory to a CD-R and file it. At this point i'm only up to about 550MB, which even includes several older versions, the acoustic mirror impulse files, sample projects, etc. so it all fits easily on one CD. This way i can easily reinstall anything whenever necessary.

Most SonicFoundry applications are downloaded as a single self-extracting .exe file. Generally this and a serial number are the only things necessary to reinstall and reregister. Most of them also have available a few additional files such as the manual in .pdf format, some sample media files, etc. These additional files are nonessential, but often very helpful. They're usually much smaller than the application file so there isn't much reason not to download and keep them.