Not every one can afford the latest and greatest.
True, but consider that they <did> have enough to buy the games, and a way better graphics card can probably be had for 30-40 dollars. But, given it used to work fine in this config and doesn't that's just a band-aid solution.
These are fairly old games and my pc is quite capable of running them easy
Not necessarily. Sure, you meet the minimum requirements, but those are generally meaningless. As Treval points out, the 5200FX sucked even when it was released.
That being said, I cannot say for sure wether this is the cause for the issues; I ran Half-life 2 quite comfortably on a 1.6Ghz P4, 1GB of RAM and a 256MB 5500FX (at 1440x900).
In an interesting twist, I found when I switched from an older generic Sound Blaster PCI to an Audigy SE that performance in everything dropped dramatically; even zdoom lagged unplayably. This turned out to be because the Audigy was not actually processing sound data, but the actual "sound card" was really just a software emulation of the DSP processor chip, since the "Audigy" SE doesn't actually have a "Audigy" chip, which was probably why it was 20 bucks.
Since "SoundMAX" is a integrated Audio Solution and a lot of integrated Audio solutions defer almost all their processing to the Processor, Combined with the fact that your working with a single core processor, you could try reducing sound quality settings both within the game as well as in your control panel. (namely, reduce sampling rate and bit-width). Try an older Audio driver.
video and audio codecs
No. That doesn't even make sense. Video and Audio Coder/Decoders are used for Decoding things like Mpeg video and DVDs. They have absolutely nothing to do with games (except maybe in-game movies).
Only other thing I can think of is a full reinstall of windows, but that's an awful lot of effort that is best avoided.
