Solid State Drives (SSD) are by all means cool: they're way faster and more effective than HDDs. Their advantages come with a solid price, but it's beyond doubt they are worth it. But what should you do with your system if you don't want to install it from scratch?
Do a data backup. You just cannot omit this step: it's playing with open fire and performing a heart surgery without anaesthetics if you mess around wth your hard drive without first doing a backup of your important data. You can copy essential files to a virtual hard drive or to Windows Home Server. If you google „data backup advice” you'll find an awful lot of tips from fellow readers. So do it!
Clean your HDD from unnecessary data. Unless you can afford a 500 GB SSD disk (who can?), there's no way you can move your 250 GB HDD onto a 120 GB SSD – they're great, but not magical. So trim: move some archives to external data storage devices, uninstall games (copy your saves somewhere, or back them up to, for example, your Steam cloud). Try not to leave more than 80% of your SSD disc's storage capacity to move – these discs work better when they have some space to spare. (Although it's true also about HDDs.) Run CCleaner – this system cleaning tool will get rid of any unnecessary files cluttering your Windows. Run the Windows Defragmentation Tool or Defraggler to defrag your system for the last time (SSD discs don't need defragmentation; cool, isn't it?).
Update your firmware. Companies that sell SSD driver work two ways: they either require you to reboot your SSD with a CD that flashes the firmware, or you just have to run a program while still in Windows (if the drive is not the primary OS drive right now). Updating the firmware is important, as the first generations of SSDs (and these disks are quite a novelty) had quite a few bugs and issues that only firware updates solves.
Aligning the sectors on your SSD. This is where things go nasty. If you clone your Windows onto an SSD from an HDD, the installation alignment will probably not work properly, and you may end up with a drive that slows down your PC's performance instead of boosting it. So when your are sure that your SSD is installed properly in your PC, fire up DISKPART. In search type in cmd, right-click on the terminal's icon and choose Run as administrator. Type in these commands in the right order, paying attention to the notation very closely – if you mess it up, you'll erase your HDD disc!
Select disk n (n is your SSD's number as provided by the previous command)
Create partition primary align=1024
Finally, moving your Windows. The safest way to clone your data onto an SSD is by using EaseUS Partition Master – a free cloning tool that will resize partitions for you and fit everything you may need. Run the program and highlight your current disk with Windows.
Select „Copy partition” (on your left) to start the Partition wizard. Choose your SSD disk (that's where you want to move your Windows). Go through the rest of the wizard – the program will guide by hand, really – and click Finish. In the main window click on „Apply” (in the upper left corner). The computer will reboot itself and clone the data.
Turn off your machine afterwards and unplug your HDD. If the Master Boot Record has been corrupted you may need the Windows recovery CD to make everything come back to its righteous place.