Anyway, the first error with Nitro was "Error 2738". This is to do with registering VBScript engine within Vista. The solution from Nitro is as follows:
The VBScript engine can be quickly registered by following these steps:
1. Run Command Prompt as administrator
1.1 Start Menu -> All Programs -> Accessories
1.2 Right click on Command Prompt and select Run as administrator
2. Type cd %windir%\system32 into the Command Prompt and hit Enter
3. Type regsvr32 vbscript.dll into the Command Prompt and hit Enter
However, when I tried this I got another error (doh!) from Windows, referring to a Regsvr32 Error 0×80004005. This was caused because I didn't run Command Prompt as an administrator - i.e. I skip-read point 1.2 above and missed the "right click" part. So there you go, I hope this stops you making the same mistake as me!