As Mark says, what you want to do is no use. The xcopy command wont copy hidden files or system files for example.
Anyway, if, as a you say, you have a catastrophe , and Windows wont start, where are you going to type this command to copy everything back onto your C drive?
I believe Windows 7 (and maybe Vista) have Backup software built in to enable you to do this, though I have not used it myself.
Acronis has been "rebadged" and given away free by some disk makers as backup software.
It is called "Disk Wizard" by Seagate/Maxtor
http://www.seagate.co.../downloads/discwizard
Western Digital also make it available free
http://support.wdc.co...addetail.asp?swid=119
Note I have never used either of these products from Seagate or WD.