Yes, you can remove your existing hard drive and fit a SATA SSD drive in its place. Best way is to get an equal size or larger and use a usb to SATA cable and Partition Magic software to copy old to new.
HOWEVER, the reason why your hard drive is at 100% is probably because it is indexing your files. Either leave it running for a day when you're not using it (reindexing will not take as long) or stop the indexing service and put up with slower searches of your data. If you don't search your hard drive often this is not a great hardship.