My Dell Inspiron 1501 occasionally does the same thing, except it comes on after 30 seconds or so.
Windows Vista Home Premium Service Pack 1 (build 6001)
Processor 1.80 gigahertz AMD Athlon 64 X2 Dual-Core TK-55
512 kilobyte primary memory cache
Main Circuit Board: Dell Inc. 0UW744
Bus Clock: 200 megahertz
BIOS: Dell Inc. 2.6.1 08/23/2006
It's driving my wife crazy who insists it's being caused by the Computer Alarm Clock I installed which she doesn't like.
Any ideas?
Kilgore