PNY CS900 240GB 2.5” Sata III Internal SSD | $30 | Amazon
The best upgrade for an old PC is an SSD, and if you don’t need a ton of space, this 240GB model from PNY is down to $30 today, an all-time low. You could even stick it in a cheap enclosure, and use it as an ultra-fast portable hard drive.
If you have a desktop with ample storage, thanks to its built-in HDD, you can use a 120GB SSD to house the operating system.