For the main database I agree with led as above however the *.mdb file could be located anywhere on your hard drive dependent upon if you selected a location when creating it from 'new'. Otherwise it should be located in this path; C:\Program Files\Seaward\PATGuard 2
However copying the database is possibly only part of your problem. Copying that file only saves your Site, Location, Asset, User, Test Instrument information. None of your "personalisation" or "customisation" is saved in the database.
By this I mean the user data, company details, short codes, logo for reports, customised reports etc. If you want that information then you need to copy a file called patguard.dat and your logo image file and any customised or edited reports will need to be individually copied over to the new PC.
Hope this helps.