My computer specs...
Home premium vista
Amd Athlon 64 x2 dual core processor 5600+, 2900 core, 2 logical processor
500 Gb memory
video card ATI radeon Hd 2600 XT
Some of the games I can play on 1080p with fill graphics without any lag. but some games the game runs okay but I sometimes get huge lag spikes like dota 2 or always get little lag like assassin creed 3(does not run smoothly) even when I lower the graphics. Which I don't really want to do, because it feels like I am playing nes games and takes all the fun out of it. I am not really a computer person. I do know about computers but not so much about the new parts. So what can I do to make my pc games run smoother.
Well, for starters, you have a very old PC. I don't think you can do much more than to reduce settings. Unless, of course, you'd like to upgrade it, but you'd have to upgrade at least motherboard, power supply unit, processor, memory and graphics card for it to be worth it. Approx ~$400-500 for upgrades. That would give you a PC which could max out most games.
new video card is needed, that card is quite old. unfortunately that will create the problem that the processor would then be very weak. And that would mean a new motherboard, memory and we should be sure of your power supply.
But providing you are happy that you will not get a perfect experience just a much much better experience a video card upgrade would be your first stop. Should you then update your other components later then you will unleash more of the power from your card.
Something like a gt650/660 from nvidia or a 7850/7870 would be a good uplift (at least twice as powerful as your current) without being overly constrained.
BUT you really need to tell us precisely what your power supply is, and preferably what your motherboard is (cpu-z can tell you).