Many thanks - success at last! I uninstalled and re-installed the Liberation font, and OOo installed sucessfully.
it immediately gave me the Error Report dialogue reported in viewtopic.php?f=6&t=44286
Still, I will just have to live with that - at least I can now use OOo.
... and I have remembered to switch the W7 security back on!
In summary, for anyone else getting the problem, it looks like it was a problem with the font files. You have to be very, very careful doing anything to the fonts in W7 as you need to be sure the registry keys are correct. As an aside, I once asked a Microsoft Windows 7 forum, staffed by MS people, how to delete and re-install my fonts folder, and they could not tell me, apart from a complete OS re-install.
To delete a font - go to C:\Windows\fonts, find the font, r-click and choose delete
To install a font - drag a copy from your backup into the fonts folder. This way, the registry gets updated properly.
If you still have problems, it could be because there are duplicate copies of the font files and you cannot see the duplicates in the Windows Explorer C:\Windows\fonts window. To see them, start an admin rights CMD window (Start > cmd and press Ctrl/shift/Enter. Navigate to C:\Windows\fonts and delete any duplicate or remaining font files (eg fred_0.ttf, fred_1.ttf etc. If they cannot be deleted, renaming them to fredzzz.ttf usually allows them to be deleted. Note that I THINK that if you have fred.ttf installed, and you install an application which installs a newer version of fred, the newer version is called fred_1.ttf etc.
I use AMP Font VIewer to manage my fonts, but it did not delete all the duplicate copies - I had to go to the CMD prompt to do so.