Ok so on some games my computer will play fine for a little while then eventually the sound becomes crackly and distorted and before I know it, it's decided to crash.

Often it goes to a blue screen with gibberish about physical memory dumps.

Sometimes I can quit before it does this but then my audio centre (realtek 97) will often come up saying there are no speakers connected.

Is especailly bad with Arma (same with the old operation flashpoint), call of duty 4 and timeshift. No problems on pro evolution soccer 2008.....

I have put down the hardware acceleration in the audio settings as this used to work for operation flashpoint a couple of computers ago - but no joy.

I'm at a loss!

win xp home
1gb ram
realtek 97 onboard sound
ati radeon x1900

You could help some by telling us what motherboard you have - but I would think the best bet is to fit a PCI sound card and see if this problem works with that too. Remember to disable the on-board card.

I have the asus a8n-e motherboard.

As I don't know where you are I cannot give you the exact URL for the latest driver for your on-board sound card - you must find the asus site for your country and locate it yourself.

Im pretty sure you can sort the crackly sound by doing:

Run
Type Dxdiag
go on sound tab
the sound accelration keep lowering it till it works

It only makes it slightly better, it'll still crash even if it's set to emulation.

It's the blue screens afterwards that wind me up the most.

Sometimes if the sound goes crackly and distorted and I manage to get out the game before my comp. crashes if i try and use say winamp, for example, it'll give me a blue screen.

Don't want to sound boring and repetitive, but have you checked that you have the latest drivers installed?
If you have and the fault persists, it is probably one of 3 things
1) Faulty motherboard
2) Wrong or corrupted driver set
3) Conflict/compatibility issue with another component.