When attempting to open (not save) an attachment in Outlook, you may get an error similar to:
This is because Outlook's Temp folder is full. Outlook uses a temporary folder for storing attachments prior to opening them, similar to Internet Explorer does. Unfortunately, this folder can become quite large, causing these kind of error messages. Additionally, there's a 100-file limit on attachments which have the same name: e.g. when opening 100 times an attachment with the same name, opening the 101th attachment will fail with the above error.
The only solution is to clean the folder. There are some manual ways to do it, but I found a tool on the internet that does everything for you. It's called OutlookTempCleaner, and is (for now at least) still freeware.