What error codes, when do they appear, does it boot, what type of mac and OS version.
A great tool to use to clean your Mac is Cleanmymac - you can try it for free and it will give you the chance to clean out old log files, language files you don't need, trash and other files.
A free tool that will allow you to see what is taking up a lot of space on your machine is GrandPerspective - it will allow you to go to the file or directory that takes up a lot of space in finder and from there delete the huge files.
They're just cache files so it's safe to delete them and it might release a couple of Gbs. Then download GrandPerspective (free) and you can use this to visually inspect the data structure on the disk and delete files (if you are sure that you know what they are). You might find that it's something like error logs that have gone nuts and gotten up to Gbs in size
jarledb:wscalioni: Another nice free option to check what is using most of your disk space is OmniDiskSweeper.OmniGroup makes great software. I am a big fan of their OmniFocus app, with sync between the Mac and iPad/iPhone. Its a great task/todo manager.
I love OmniFocus although it's too expensive. Perfect for Apple devices but it's not multi-platform. Todoist is serving me well.
Going a bit off-topic here though...
askelon: I think its running snow leopard. I think I will just reinstall it from scratch at this stage. Is there an official way to download the iso's? I couldnt find any other than the dodgy torrent ones for it. Pretty sure apple only had a link to rebuy them on CD.
Where are you located, someone near you maybe able to have a look and I've you a hand.