You have several options:
- Use the cloning program that you can download from your
disk manufacturer's home site.
- Use an imaging tool such as Acronis or Ghost.
- Temporarily install both disks as slave disks in some other
WinXP/2000 PC, then use xcopy with the appropriate
switches to copy Windows across.
- Boot the machine with a Bart PE CD (ww.bootdisk.com),
then use xcopy as above.
The last two methods may require some minor fixes to
restore the boot environment.