If you mean you're building a new computer and want to just use your current hard drive and installation of Windows and applications (instead of a full reinstall) then it's not a very hard thing to do, and it's much faster than reinstalling.
First make sure you have the installation disks for all your hardware. Or at least have unzipped copies of the newest drivers downloaded from the manufacturers' sites (make sure they're uncompressed and in their own directories, so that you can find them during the first bootup). All you need to do is boot into safe mode, go to the Device Manager, and delete every item in it (unfortunately that's a one by one thing you have to do).
Then shut down, swap the hard drive into the new system along with any hardware you're migrating, and boot. Windows will detect your motherboard and all your hardware as if you've done a new installation.
You have to be somewhat logical about the hardware though. If you've got say...SCSI CD-ROM's, then make sure your drivers are all on the hard drive, not on CD, since you won't have access to the CD until the SCSI card drivers get installed.
Doing it this way isn't the absolute best way, but it works fine most of the time. The only possible issue is that there could be leftover drivers from the old setup, but most of the time it won't be a problem.