It seems that hard drive space is not really your issue, keep about 15% of the hard drive open for defragging and just overall system performance.
There are lots of issues that can contribute to system slugishness. Please tell me what kind of antivirus you are using, what you have starting up on your system and also how much CPU usage you have and Processes ( you may have lots of things starting up when you boot the machine )
If you are looking for an external hard drive this would be a good alternative to your current drive:
I have had no issues with the Passport drives and they are great!