why do I sometimes get a blue screen with bunch of numbers on it after
I boot up the system.  At the end it says "physical dump of memory. 
Please contact system administrator".  When I turn off the power and
turn it back on, The computer boots up fine and windows would come up.

Hi, 

That is called the "Blue Screen of Death", really, that's the
technical term for that screen.

Its a critical error screen and means that the operating system could
not work around the problem, whatever that was, and has halted.
Normally this happens on boot, as you describe, but can happen at any
time the system runs into a problem the error checking code can not
deal with.

The causes are numerous, and there are many helpful websites,
including Microsoft's Tech area that can help you find out why this
particular screen is coming up.

Links of interest. 

Microsoft TechNet -- Demystifying the 'Blue Screen of Death'
http://www.microsoft.com/technet/treeview/default.asp?url=/TechNet/prodtechnol/winntas/tips/techrep/bsod.asp

XP registry bug causes blue screen of death
http://www.theinquirer.net/15060201.htm

Blue Screen O' Death
http://www.ntbrad.com/bsod.htm

blue screen of death - a searchWin2000 definition
http://searchwin2000.techtarget.com/sDefinition/0,,sid1_gci214376,00.html

Blue Screen of Death -- The Haiku
http://www.tuxedo.org/~esr/jargon/html/entry/Blue-Screen-of-Death.html

Query on Google

+"Blue screen of Death"

A Hint : Use that query adding the operating system you have and the
error code the screen is giving you, after it on Google. This type of
help is on the Internet in mass supply and you'll probably find the
exact nature of the problem.

Thanks for the question. 

webadept-ga