Vista 32 to Win7 32 or Vista 64 to Win7 64 = Very Easy
Vista 32 to Win7 64 = Not as easy
Which way are you going?
Open the Start Menu, right mouse click on Computer and choose Properties to find out what version of Vista you have installed.
If you haven't done so you probably want to run the Windows 7 upgrade advisor just to confirm you're good to go ahead. Download Windows 7 Upgrade Advisor