Hi there garethmanuel,,,
All what I can say in such a case is ,,,,,
Firstly, check if the drivers of the NVIDIA really support windows7 but since it was working as you mentioned it is highly lickly that there is no issues with the drivers .
Secondly,, do you have an AV (Anti-Virus)( with an engine which supports win7) Installed on your PC ???? the second possibility is that there may be a malicious software on your PC .e.g a Virus , trojans etc. you have to veryfy this .....
Finally, if you veryfy the above mention things I may recommend either a reinstall of the OS which will not take more that 26 Miutes or to get the official version recently launched by MS and install it then check .
Best Wishes