In reply to The Lemming:
Join the 21st century and start filming and editing in 30/60fps. The footage filmed in 25p will look better in a 30p timeline than doing the inverse so you can sort of solve your current problem by changing the timeline's properties and making it 30fps.
If you do want to slow down the 25p footage so that it matches the frame rate there's normally an option accessed by right clicking the cllip that says 'Interpret as' or something like that depending on what software you are using, and gives you options of frame rates, so if you right click the 30p clip you would select 'interpret as 25fps'
On Vegas and few other software the options are under the clip's properties and you change the 'playback rate' or 'undersample rate' to suit