Allright. If the computer gets confiscated, they will most likely come when you least expect it. They do this on purpose. So if you have a window of opportunity, you'd want to take care of it right now.
If it's confiscated they will send the computer down to the nearest high-tech crimes unit, and there they have phorenzic specialists that go through each byte of data in your computer. Even if you deleted it it can be traced.
Seriously, your best bet is to reformat the hard drive, or if it's a removable hard drive.. carry it with you or hide it in the woods.
To reformat the hard drive is to completely swipe the whole entire computer back to square 1. Windows won't even be installed. So you're going to want to back up everything important, and throw it on a cd.
Check to see if your computer came with a Restore cd of some kind (should have all the default drivers and programs that orignally came with your computer) this keeps it simpler, so to put windows, etc. back on your computer you just toss the cd in.
If you don't know much about computers, I think you'd be better off having someone else reformat the hard drive or pay for it to get done (as suggested above), it's really easy to get stuck.
But, if you insist on reformatting it:
1. You need to get into the dos prompt. Newer versions of windows have a built in dos console, but you aren't going to want to run that because windows is still active and running when you open it, making it impossible to erase windows. (You can't erase it if you are in the middle of using it).
So one way of getting to the REAL command prompt is by grabbing a system boot disk (probably came with your computer. Pop that into your disk drive, restart windows. It'll ask you if you want to load the command prompt with cd support, go for that.
Or, if you can't find a boot disk (in windows you can make one also) You can open up the safe mode dialog and use it to get to the command prompt.
Restart your computer, hold down shift and press f6 (or maybe it's f8?) , it should open up the safe mode dialog. Select that you want to open up the command prompt.
If that doesn't get you there, you can try triggering windows built in safe mode mechanism. Restart your computer. When the Loading Windows screen appears, turn your computer off, and restart it again. This should bring you to the menu.
Once you are in good old Dos
type:
format C:
This will erase everything. It'll ask you simple questions. Respond to them. Once it's wiped clean, try putting in your system restore cd or whatever that probably came with your computer. If you don't have it, try putting in the windows cd to reinstall it. Reinstall all the drivers after that, yadda yadda. have fun.
Eat. Sleep. Breathe. Ski.