My friend is having trouble running Team Fortress 2 due to a "cannot read memory" error. Any ideas?

[Lordgeorge16]

So I was helping my friend install a new 2GB stick of RAM in his computer the other day (He was originally just at 478MB. Seriously). Took us a little while, and after my thumbs were nearly bleeding, we finally got it in. We started up his computer, let everything install properly, and he was good to go. So we tried out Team Fortress 2 after waiting for what felt like hours for it to finish downloading. We launched it, and after a few minutes in-game, it gave him the following error.
[quote=error message]The instruction at "0x164c03a2" referenced memory at "0x1794c42c". The memory could not be "read".[/quote]
We have no idea what it means, neither of us have ever seen this error before. Just to be safe, we tried out a couple of other games. Half-Life 2 ran just fine, and Portal 2 didn't close out on him like it used to. Now, Team Fortress 2 used to work fine for him, it was just extremely laggy, partly due to his lack of RAM and the other reason because of all the stupid hats and shit. Now it won't run at all.

Any ideas on how to remedy the problem?

My friend's computer info:
GPU: ATi Radeon Xpress 1100
2018MB total RAM (Approximately)
OS: Windows XP Home edition 32-bit
CPU: Intel Pentium 4 CPU 3.00GHz (2 CPUs)

[Drsalvador]

Have you tried it again lately? Often this happens in source games, even with the best PC's and is more often than not a source error.

[Lordgeorge16]

[QUOTE=Drsalvador;34832320]Have you tried it again lately? Often this happens in source games, even with the best PC's and is more often than not a source error.[/QUOTE]

I just had him try it again a minute ago, still got the error.

[naelstrof]

Validate game cache files via properties.

[Shadaez]

is this a BSOD?

[Lordgeorge16]

[QUOTE=Shadaez;34835477]is this a BSOD?[/QUOTE]

No, it just closes to the desktop and an error window pops up.

[tratzzz]

Tried running Memtest?