Okay so last night I decided to run the memory diagnostic and chose the extended (or expanded can't remember what its called) but it was taking to long so I stopped at about 20%. However now my computer seems to be running slower then normal I havn't done much since then. Would running memory diagnostic and letting it run all the way help?
No it shouldn't unless it was in Windows and didn't flush out the memory it used. If it was and you restarted, it should be fine.
If its running slow it could be clutter, as corroded pointed out, or a failing HDD. Just had one at work. It passed our diags but XP was very slow. And when I cloned it to a new HDD and ran a write test, the HDD failed that and the enw HDD ran smooth as butter, like XP with 1GB of RAM should.. Remember your OS will use part of the HDD for the pagefile and if the read or write is going bad on your HDD (bad sectors) it will be slower since it will keep trying to read/write.