You should only need the latest patch. Are you shutting down all non-essential programs before starting the game? There are several critical and gaming related patches for Windows XP.
Unique and quest items do not show up in the trade window, so can not be sold. Unique items have their names changed on act change, and are then sellable.
If it is a hardware problem;
The hard drive manufacturer will have a diagnostic program on their web site, which would be more thorough than Scandisk.
There are a couple memory testing programs you can try;
Memtest86 and
MS Windows Memory Diagnostic