So after a weekend of banging my head against a wall, I have finally put to bed the issue of CRM Client not installing on a Vista Business laptop that has had a pre-released version of .Net installed on it... and to be honest it was a simple fix in the end!
Once I had got the .net uninstalled, it was still telling me that it could not install the client, so I took a backup of the registry and started messing around with the .net and ASP keys, which did provide fruition in the end, however if you are going to do this please, please please take a backup other wise you could be in a whole world of trouble!
So once finished experimenting, i simply deleted the entire ASP key from HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft
Then rebooted the machine and carried out the install/config and guess what, it all worked!
So good luck if you have this issue and hope that this helps!