To reset your homepage:
in IE/Safari:
go to the IE/Safari menu (far left tab) and choose "Preferences"
the first form should have on it "Homepage". Put the mumsnet URL back in.
in Navigator:
go to the Navigator menu and choose "Preferences"
On the next screen choose "Navigation"
In the "Homepage" field, enter mumsnet's URL
Other browsers should be variations on this -- go to the application menu, choose "Preferences", then look around until you find the "Homepage" field.
To avoid the site popping up on your address bar, you can empty the cache and history -- note that this will put you back at square one for autofilling the address bar or using history:
In IE: go to the IE menu, choose "Preferences" then choose "Advance" then hit "Empty Now" under cache and "Clear" under history
In Navigator: Navigator menu > Preferences > Navigation --> History , then empty cache and clear history
In Safari: Safari > Reset Safari (seriously resets everything and deletes cookies you'll get a prompt)
I use OSX, so yours might be variations of that.
Re the porn sites: I've accidentally logged into two in recent weeks. One used to be a recipe site (they let the registration lapse and some porn outfit bought the domain name) and the other was while trying an obvious address for a ski company (the actual address was very very close to the one I had typed). So it could easily be accidental. Those sites sometimes autoload others, one of which might have reset your homepage to it. Argh!