One possible fix used for laptops may help you there.
[SIZE=+3]Notebook has STANDBY button "greyed out":[SIZE=+1]after Windows XP Home recovery.[/SIZE][/SIZE]
1) Click the "Start" button in the bottom left hand corner of the screen.
2) Select the |"Control Panel"
3) Click the "System" icon within the "Performance and Maintenenace " section
4) Select the "Advanced" tab across the top
5) Click "Settings" within the "Performance" section
6) Select the "Advanced" tab
7) Near the bottom there is a "Change" button, click this
8) Within the "Virtual memory" section there is a button "System managed size"
9) Click this and then the "OK" -- 3 times, to exit the settings.
Restart your machine and STANDBY is once again an option.
http://www.fujitsu-siemens.co.uk/rl/servicesupport/techsupport/lifebook/General/RrestoreStandby.htm
Another thing to look at is to see how much drive space is available. XP requires at least 1,8gb minimum be available for swap files.
