You dont clean the Bios, you can reset it to the defaults or you can usually update it, but thats not recommended unless it needs a critical update, so many people have tried it with bad results, like the pc not starting up after an update, but it is highly unlikely that anything has changed in there so I would leave it and concentrate on the actual computer hard drive.
You can clean you hard drive if you right click on the C: drive icon in My Computer. Then click on Properties, then click Disk Cleanup. There is also on option to clean all but the latest restore point, that saves a lot of hard drive space.
You can also defragment your hard drive which sorts all the files out so that when a file is accessed it finds all of it at once, instead of finding fragments and putting them all together. Files are not usually stored all in one but are spread around the drive, taking longer to find the fragmented bits. It also consolidates all the free space for faster writing to the drive.
You can also go in to MSCONFIG and click the Startup tab and see what programs are running. Anything in there automatically starts running as soon as Windows starts. You can dissable the ones that you know don't need to be running. Not things like Antivirus, Firewall or anything to do with Internet though.
Go to the start menu and type msconfig in the search / run box and press Enter. When you have unticked the ones you dont want running restart the computer.
You will probably get a box pop up when you have restarted saying that you have been in to the configuration settings etc. Just tick the little box on the left that says Don't show this again or whatever it says.
You could also uninstall any programs that you don't actually use.
Go to the Control panel and click on Add and Remove Programs for Windows XP, or Programs and Features in Windows 7.
If it's just got slow gradually,cleaning up usually helps a bit.
If you just want it to be a bit faster then maybe it can have more Ram fitted on the motherboard, that helps, also upgrading the processor, and a faster hard drive will help.
You can also use a little program called CCleaner
whhich searches the hard drive for all the rubbish left over from uninstalled programs, unused files, temporary files that are no longer needed etc. It's a free program and very popular.
You can also clean the registry which gets rid of a lot of rubbish left in there. There is an option in CCleaner whhich will do that.
Hope this helps.