Without knowing your computers vital statistics, we can only offer generic advice (good advice yes, but very generic in nature) that may not in fact deal with your specific issues.
As an aside, "nuke & pave" (reinstall / reformat) will get you back to the starting point, but once you start adding programs again, all bets are off.
Can you give us things like:
What processor do you have in the machine?
How much RAM do you have? (and do you know how much you can add to the machine?)
How big is your hard drive? (I know you said it's 50% full, but depending on the age / size of the drive that may be a more critical piece of information)
What video card do you have in this machine?
What operating system are you running?
That will enable us to give you more specific advice tailored to the machine you have.
NefCanuck
As an aside, "nuke & pave" (reinstall / reformat) will get you back to the starting point, but once you start adding programs again, all bets are off.
Can you give us things like:
What processor do you have in the machine?
How much RAM do you have? (and do you know how much you can add to the machine?)
How big is your hard drive? (I know you said it's 50% full, but depending on the age / size of the drive that may be a more critical piece of information)
What video card do you have in this machine?
What operating system are you running?
That will enable us to give you more specific advice tailored to the machine you have.
NefCanuck