Kipper,
Sometimes when Photoshop starts behaving weird or erratic it can often be sorted by resetting the preferences file.
To do this hold down Ctrl+Alt+Shift (Cmd+Opt+Shift for Mac) as Photoshop is starting. You will be presented with a dialog box asking if you want to delete the preferences file.
The other option is to manually delete the file. It has a .psp extension and is usually found in "Documents and Settings\username\Application Data\Adobe\Photoshop\version#\Photoshop". The filename is usually along the lines of "Adobe Photoshop ?? Prefs.psp". This file may be hidden.
You'll have to go through preferences and re-enter your preferences, but I would bet that this is a lot quicker than trying to troubleshoot the problem.
Whoops... Too Slow!
Hope this helps.
Cheers
Matt