When I try and isntall D3 it fails due to lack of space. I initally tried to install D:GamesDescent 3, that drive has 350Gb free. I then tried to the default directory on C: which has 109Gb free but both failed.
Attached a screenshot of error. Any ideas?
Edit
Oops didnt notice there is an option for setting as problem. Don't think I can change it now.

Sounds similar to the experience others had trying to install Giants: Citizen Kabuto. They've been able to solve it redownloading the installer until it worked fine.
Hope it may work for you too

Thanks. I'll give that a go.

Unfortunately no difference.
Although it did seems to get further into the install.
I wonder if it would have anything to do with using a download manager. After a while I start seeing invalid password errors, although file keeps downloading. Ill try downloading it without the download manager later.

Sorry to hear that.
I don't know if using a download manager may be the cause, I'm not using one (just Firefox 3 vanilla download functionalities) and haven't encountered the issue so far, but 11 games are not a number high enough to make statistics.
It can be worth a try, if you don't have download caps on your line.
If it may be of help checking your file integrity, the result I get using 7-Zip File Manager "Calculate Checksum" functionality (File -> Calculate Checksum) on the file 'setup_descent_3.exe' is:
Files: 1
Folders: 0
Size: 836244288 bytes
CRC checksum for data: 33EC3D5F
CRC checksum for data and names: E286770B

This might be a stupid question, but have you tried installing as an administrator yet?
sometimes, the disk isn't writable by a program Vista doesn't recognise as an installer.
I ran it as an Administrator, and it installed fine. I'm even using a 64-bits version of Vista.

Well I downloaded it again without any download managers and it installed fine.

Nice to hear it worked in the end. Have fun!