First off, do you have another monitor you can connect to your computer, and do you have another computer you can connect to your monitor (or do you have friends that could help)? This would narrow down which device has the problem.
When you say it "gets no signal", does it show a mesage about "No Signal", or is the display simply black?
As for the drives, did you just copy your files from the old HDD to the new one, or did you simply install the new one as a secondary drive, and Windows is still installed on the old 40GB drive?
A "no signal" warning from your computer shouldn't be the result of your windows installation UNLESS Windows is trying to drive the monitor at a resolution that it cannot handle (i.e. a 15-inch monitor won't run 1600x1200).
How far do you get when you boot-up the computer? Does ANYTHING display on the monitor? Do you get messages from your BIOS about memory and drives installed? Do you see the Windows XP Boot screen (with the scrolling bar underneath)?
If you can see anything after powering-on the computer, try booting into Safe Mode. Reset the PC, then press F8 about once a second until you get to a screen that lets you select Safe Mode or Normal Mode (if such a screen never appears, thats a problem). If you can boot into safe mode, do you see your desktop? Try setting your resolution to something low (like 800x600). Then restart your computer and let it boot normally, did this fix the problem?
If the monitor never displays anything, re-check the cables (at the monitor and the computer). Do you have on-board graphics or a discrete graphics card? If its a discrete card, turn off the computer, then check that the graphics card is properly seated in the mother board. If the card requires a power connection, make sure that the power cable is plugged in tight.
Trying out these things should either resolve the problem or at least narrow down where it might be in your system.