HP Pavilion slow to wake up from screen saver or stand-by

HP Pavilion a1250n, Windows XP Media Center edition
(SP2), Athlon 64 X2 dual, 1GB ram.

About a month ago, the PC suddenly began taking a long
time to wake up from both the screen saver and standby
mode. For 30 seconds or so, there is just an empty
blue screen with an hourglass for the mouse pointer.
Then, after the user profiles appear, the mouse still
isn't enabled - it takes another 20 seconds or so.

I can't recall any system or application modifications
I might have made back then. What might account for
the behavior?

Thanks in advance.