Can't standby! Help!

I have an IBM Thinkpad R40 with Windows XP pro. It has a 1.3ghz Pentium M, 512mb of DDR ram, 40gb hard drive, ATI Radeon 7500 32mb, and a (bought by me sperately) Netgear 802.11b Wireless PC card.

My laptop used to be able to sleep, but recently (last few days) when I go to standby, it just hangs on the blue screen that shows the Win XP logo and says "Your computer is preparing to standby" or something like that. It never enters and I just hard-reboot.

Any idea what this could be caused by? I haven't done anything to my computer in the last few days that I can remember.

Thanks.