Quick installation question

Hi

I have a Dell laptop on which I want to install Windows 7. This is because I'm told it has (or has had at some point) a backdoor trojan on it. To complicate things, it's running an unauthorised copy of Windows XP.

So... the recommended solution is to wipe everything (I have backups of my files) and do a clean install of Windows 7.

However I'm not sure how to best go about this.

I have another clean laptop - should I download the Windows 7 installer onto there, and put it on a USB flash drive? I've just read the thread about that - Windows 7 USB/DVD Download Tool[2]=General%20Tips - and it seems to make sense.

Does the Windows installer download as a single file/package that I can then copy to the flash drive? Or do I have to select that as an option somehow?

And how big does the USB flash drive need to be?

Finally, should I totally wipe/reformat the hard drive on the infected laptop, before I install Windows 7? Or will the installation totally reformat it anyway?

ANY advice appreciated, probably quite obvious I'm lost and a bit of a noob :)

Thanks in advance
Beyond Thrilled

Use your other computer to create the 4gb flash stick installer using the tool you have found.

Boot the flash stick using one-time boot menu key given at bootup; it may be found under Hard Drives. Or set the connected flash stick to boot first in BIOS setup.

On second screen, select Repair My Computer, click through to recovery tools list, open Command Line, type:

DISKPART
LIST DISK
SELECT DISK # (replace # with target HD)
CLEAN ALL (will write zeroes to every sector; takes a long time)
EXIT

Now reboot installer, select Custom Install, create New partition, Format before installing.

Many thanks for your help guys. I've decided it's best to buy the Windows 7 installation pack rather than download it, so I can install it straight from the CD.