|If the main chipset drivers for your mboard have NOT been installed, then installing them will probably provide the info Windows needs to know about how the main chipset supports ACPI features, including how it supports Restart, Shut Down,and Hibernate modes, and then Restart should work fine.|
(Whoever reloaded your computer SHOULD have already done that, but maybe they didn't.)
Go to the Dell site and download the specific main chipset drivers for your specific model and install them.
XP doesn't have the drivers built in for most things that first came out after XP was first released, circa 2001, and it doesn't have some of the drivers built in for things made before that.
Whenever you load Windows from a regular Windows CD (or DVD) from scratch, after Setup is finished you must load the drivers for the mboard, particularly the main chipset drivers, in order for Windows to have the proper drivers for and information about your mboard hardware, including it's AGP or PCI-E, ACPI, USB 2.0 if it has it, and hard drive controller support. If you have a generic system and have the CD that came with the mboard, all the necessary drivers are on it. If you load drivers from the web, brand name system builders and mboard makers often DO NOT have the main chipset drivers listed in the downloads for your model - in that case you must go to the maker of the main chipset's web site, get the drivers, and load them.