Hi j_palm,
Two common paths for installing a new SSD are:
1) If you have a Windows Installer Disk and your Windows Key you can install a fresh operating system on your SSD. From there, you can have both drives plugged in, and as long as you boot from your SSD you will still have access to all the data still living on the older drive. (This isn't ideal, as your old OS is still taking up room on the HDD, but if you don't have an external drive it is your best option)
2) Move all of your data (execpt the actual operating system) to an external backup drive, then once you had no more than 100GB left on the HDD clone it to the newly installed SSD. From here you can re-format your old HDD drive and transfer the data from the...