Your computer is probably old and has been in use for a long time. At a very basic level I'd recommend using a drive cleaner like CCleaner, and if that doesn't help we can continue from there.
@Lukasz if you can upgrade your os then try and do that but avoid Linux as its not that good of an os for normal everyday tasks but if you cant then go through your system and delete files and assorted other things you dont use. also open a task manager and end non needed tasks and applications.