Windows 7: sort of freeze .... ??

I'll try to explain.
Not my comp so vague on some facts!
Windows 7 Home Premium upgraded from Vista Home.
Vista was running OK & 7 did for some time too.
Dell Studio Hybrid 140g.
Intel Core 2 Duo CPU T5750 @ 2.00GHz.
Windows 7 is 6.1.7600 build.
This unit comes with a wireless keyboard & mouse that somehow talks to the computer. (nothing in USB)

Problem... VERY frequently becomes unresponsive to mouse.
When booting up it gets to the password window & I can type in the password but even though it APPEARS to 'get' the mouse click (color change of button), nothing happens. Hitting ENTER on the keyboard is accepted.
Going along fine & then mouse has no effect & the windows 'freeze'.
The windows key brings up the start menu whilst in this state. (keyboard works?)
I've discovered that if I ctrl/alt/del & select START TASK MANAGER, the freeze immediately ends even before I try to end anything.
Never get a blue screen.

This comp has no PS2 ports. A USB mouse does the exact same.

Clues & questions welcome!!!