I went thru the not showing up also w/ GTA IV, and I actually set my copy aside for a year. It's hard to make it work, but it's possible. just need to give it all it wants like the .net framework and a couple other things need to be done but I can't remember specifics, it's online tho... unless you mean the disc not showing up, you should be able to get the game to show some signs of life. I did get it going, but I had to get a better video card. I had a Geforce 8500 GT, and upgraded to a GTS 250. it's a cheaper Palit brand card, cost $130 shipped from newegg.. bottom line tho, unless you upgrade your video card and add a gig more ram, it's not worth getting the game running because your computers not going to run it. also don't know your prosessor speed, but unless it's a dual core of at least 2.4 GHZ, it's not going to handle it.. my computer is an Athalon x2 6000+ which is dual 3.1 GHZ, and altho it's fast enough and runs the game fine, according to
Can You Run It, it's not a reccomended prosessor.
as for PS2 games, not sure what would cause a disc to be truly bad other than a damaged foil or heavy scratches. but it does sound like your PS2 laser might be out of adjustment. used discs from gamestop ect.. if they have no scratches or scratches are barely visable, they have been resurfaced and were once very beat discs.. and resurfaced discs can read diffrently than originals, but the PS2 should be able to read them unless the laser is out of adjustment. resurfacing makes it harder to seek the lead track in the disc, tho it is still readable.. my PS2 is very finikey with resurfaced discs.. my copy of GTA: SA is weird. I got it used ( resurfaced ) and I have to put it in the machine, turn it on, wait for it to try to load, open, rotate disc 180* reinsert, and then if it doesn't work, use another disc, then it'll work. I have the original 1st gen brick PS2, with the laser read issue. I know the laser is falling out of adjustment, but I have been lucky and never needed to open it up yet. all my games still work, and all that I bought new work flawless, so it could appear that if I get a used game that does not work. I could see it as a defective disc, altho it would most likely be my machine