Can anybody tell me how to stop my computer being able to access adult sites, i use xp

How do you mean stop the machine accessing adult sites?
is your machine always being redirected to these adult sites, or do you want anyone using the machine not be able to access these sites?

i want to stop anybody using the machine accessing these sites

you can set your PC if you're using IE by opening IE going Tools ...Internet options ...Content at the top you'll see content advisor ....Click on Enable and you can set language, Nudity, Sex and violence there.

when all set to 0 a lot of sites wont open

you can buy filters for your PC.

a few schools etc use this one http://www.netnanny.com/

but before you buy read all the info about it.. as it says " staff at Friends University reported that the software blocked information on sexually transmitted diseases, the politics of prostitution, and even a site about Adam and Eve."
http://www.peacefire.org/censorware/Net_Nanny/

have a look here
http://www.net-consumers.org/erica/guide.htm

your ISP may have some filtering blueyonder offer pcgaurd http://www.blueyonder.co.uk/blueyond...ecuritypcguard

And if I use Mozilla Firefox?!

I don't currently have Firefox installed, thus I can't experiment. If Firefox doesn't have an adequate filtering tool,,,,,, I found the following through Google;

http://www.freedownloadmanager.org/d...ilter_34998_p/

It's Freeware. I cannot attest to its performance, it's merely the results of a search. It claims to work on all browsers.

Here's a more general jump off point;

http://www.google.ca/search?hl=en&q=...rn+sites&meta=

In researching this, I have read that some of these "tools" will also block things you may want to see, such as information on sexually transmitted diseases, etc. Things which may be of an educational perspective. Thus you will probably have to "fine-tune" any filtering which it performs, or, dis-engage it for certain endeavours.