The very first thing you need to do is BACK UP YOUR DATA on something. Then BACK IT UP AGAIN on something different.
Then the question you have to ask is do you need storage space, speed, or reliability?
You only get to pick two
If your PC is about 10 years old, you probably have a SATA 2 controller which will top out at about 280 Megabytes per second in real-world useage. Just about any SSD (solid state drive) made in the last few years will be not only able to make use of it's full potential - but actually be limited by it.
I'd pick up a WD Blue 500Gb SATA 3 SSD for about $60 online. They are
not the best drives - but for a 10 year old machine they are more than adequate. If you want to spend a bit more grab a Samsung - they will generally perform better if you take that SSD to a newer, more modern PC if you upgrade later.
I would avoid replacing your system drive with another traditional HDD (hard disk drive) as they are simply too slow these days. If you have never used an SSD based system before - you will probably be amazed at how much faster the computer seems.
Having said that -
HUGE hard drives are available for really quite cheap these days - and as long-term large storage they are very useful for putting stuff on that doesn't need to be fast. Movies, photos, music, etc - plonk all that on a 4Tb drive for next to nothing.
Your 10 year old system may not be able to
boot from such a large disk though, so make sure it's just used as a storage drive and not the one you attempt to install the OS on
Whatever you do -
don't buy a used or even a reconditioned hard drive or solid state drive. Your data is worth more than any savings. And keep backing everything up!!!
Best of luck to you!