Windows 7: mouse pointer keeps changing in adobe photoshop cs5

hey all, this is a weird problem, its starting to get pretty annoying. Sometimes after ive been in photoshop for awhile my mouse pointer keeps changing from whatever tool icon im using (brush, marquee, wand, whatever) to the standard windows mouse arrow and back again, constantly cycling through them.



any idea what might be causing this?
its only just started happening recently.