My son works as a Dell maintenance man. He was not hopeful for your problem, other than a reinstall. Hwever, he offered these alternatives. All three will require that you can, at least, get into Safe mode.
First, if you have it, uninstall SP1.
Second:
Link Removed due to 404 Error To make hibernation unavailable, follow these steps:
1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.
4. At the command prompt, type powercfg.exe /hibernate off, and then press ENTER.
5. Type exit, and then press ENTER.
The third, a little more tech:
The best workaround so far is to change some settings for the wireless network adapter:
http://www.dellcommunity.com/supportforums/board/message?
Open Device Manager (type Device Manager into the start search box) and modify the properties for the "Dell Wireless 1395 WLAN Mini-Card".
Leave the device enabled, but open its properties, go to the Power Management tab, and uncheck the "Allow the computer to turn off this device to save power" checkbox. The reboot problem should disappear. You won't need to uninstall Vista SP1, nor will you need to disable Hibernate. Some users have noticed that with this option unchecked, the battery tends to drain more overnight than when they had the option enabled.