Might be worth giving the tech support section of the Techspot site a go. I managed to sort out a fairly obscure OS issue with some valuable input from one or two of the guys there. There’s also a couple of extremely dedicated peeps that frequent the Virus/Malware section for anyone that suspects the presence of unwanted/stubborn guests on their personal computar mashine.
A few random thoughts -
I’ve had an issue with a failing HDD only once before. Bad sectors would show up under chkdisk, files would go missing at random and occasionally the system would start to crash. Eventually the boot sector packed up and OS go bye-bye. If the issue with Steam is specific to Steam only then it may signify something else.
Bad RAM? Give Memtest a few runs.
Wonky CPU? Try running a stress test in Prime95.
GFX card stability? Try Furmark.
Might be worth running some tests specific to individual components to narrow things down, but if the list of issues is as substantial as you say, I’d be completely guessing!