What isn't running as well as you'd like, or what do you know you cannot run but want to?
If the need is slight, you could add a better graphics card. A GT1050 does not need a PCIe power cable and should run in your system, boosting graphics a visible amount. This money would not be wasted, as you could use this card if you later decide to rebuild (although you may want still stronger; we really need to know what you want to improve or be able to run that you can't now).
For the most part, however, I agree with those suggesting to try to rebuild the entire system. What you have is old enough that some parts (e.g. motherboard and PSU) may be nearing their physical end-of-life (typically due to capacitor aging and/or tiny but cumulative stress from thermal cycling).
A 120GB SSD is pretty small, even for a limited-purpose system.
For a near-term, easy upgrade, Vapour's suggestion is not bad, but you'll also need to figure in the cost of a Windows license, and I really think you'll want a larger SSD, preferably 480GB-512GB. The integrated graphics of the 2200G is considerably stronger than what you have, and you can add a graphics card later. He doesn't specify a new PSU for you, which may not be needed except for the previously-mentioned capacitor aging, and that you may at some point want to add a more powerful graphics card, likely to require a larger / better PSU than what you have; what is the brand and model (not just wattage) of the PSU you have? Does it have any PCIe power cables on it?