Follow these instructions, exactly as written. Leave nothing out. Take no shortcuts.
Remove Reader with Add/Remove Programs. If a message appears that says it couldn't remove all the files, click on Details, write down the names of those files, then hunt them down and kill them. Remove any and all Adobe, Acrobat, and/or Reader folders from your system.
Log onto the internet, copy
ftp://ftp.adobe.com/pub/adobe/acrobatreader/win/5.x/ rp505enu.exe into your browser Address window. Click the Go button. When the download screen opens, click the Save button and save the file to a folder on your hard drive you can remember. When the download is complete, log off the internet and reboot your computer into SAFE MODE. You do this by holding down the F8 function key while the system reboots.
Go to that folder you remembered, and double click on the RP505enu.exe you find there. The rest should be self-explanatory. When installation is complete, reboot back into normal mode and enjoy your Reader.