Hardware or software problem?

[Snuffeldog]

Hi all,
I'm not sure this is the right forum to post this problem, but the main problem is about networking so I think this is the best place.

In fact I've got a lot of problems wit this pc, the target is to reinstall windows xp but first I want to know for sure that all hardware is working properly.
My pc is a Dell Dimension 8200 and XP SP2 is installed on it.

These are my issues:
I have a very bad ping rate (about 25%) when pinging to router.
I can't ping to an external ip-address.
DHCP doesn't work, however after boot, he has assigned himself a correct ip-address.
The pc isn't able to resolve dns.
I used wireshark to analyse traffic during pinging to te router and noticed that my pc was able to send all packets and the router replied on all of them but my pc only received a couple of them.

Other strange issues are that I can't boot from floppy, that I can't boot linux live cd's (I was however able to boot the xp cd, a MS-DOS boot cd and a homeserver boot cd) and at last the most strange of them all:
when I try to read a cd, an usb stick or a floppy from windows all the content of textfiles is kind of scrambled and executables are corrupted. The i386 folder I had saved on my harddisk has the same problem, even when I read it on another pc.

solution attemps
I have tried 3 network cards.
I have reset the bios.

All this problems suddenly started a week ago. I have been searching google for days now and I've tried everything I came up with so I'm getting pretty desperate now, does anyone as a clue about these problems?

Greetings,
David