Lightroom 2.7 cannot launch any application

Question

Lightroom has worked flawlessy on my system (XP64) for a long time, but now it shows a number of issues that, I think, share the same cause:
when I try to open another catalogue, LR exits and nothing more happens. When I subsequently restart LR, it opens with the desired catalogue.
when I request "show in explorer" I get the message "[Explorer] Windows cannont access the specified device, path, or file. You may not have the appropriate permissions to access the item." This is on a local file while being administratror.
when I try to access the printer settings, nothing happens.
when I try to edit in PhotoShop, nothing happens.
when I try to print I get the message "Could not start print job" and the progress bar top left remains there for ever. However, I can print, provided I first activated the printer driver software from another program. E.g. only accessing the printer settings from notepad is sufficient to make the printer accessible.
I looks tro my as if LR is contained in a kind of sandbox. Regrettably, turning off any virus protection, firewall and anti-spyware I can find does not help. I twice ran the repair option of the installer and then removed and reinstaled the program. Still.
I could do with some advice, therefore...
thanks,
Wim

Answer 1

I have found the culprit: it's Comodo security, to be precise: Comodo Defense Plus. I do not understand how this can be, for the first thing I did was completely exiting Comodo. This didn't solve nor alter the problem. But when setting aside the settings file as Lee Jay suggested, I noticed some activity in the Comodo icon and against my better judgement I had a look again. I found Defence Plus in "training mode"; setting it to "disabled" immediately solved all lightrooms problems.