Well, you could try to change the environment variable for the temp folder (try echo %temp% in a command window to see what it is now), but if it's a badly written installer program that won't work.

It might be better to free some space on the c:-drive. Things to try:
- ccleaner in its default settings
- disable System Restore and enable again: this cleans the system restore area. Might easily give you 5 GB of space.

Kees