OK my computer has a single 55 gb harddrive and i have a new one i recieved from a friend that is a 20 gb. I want to add both of these to my computer and have them run together is this possible. If that isnt possible how do i have it so i can choose which harddrive i want run on when i start the computer. Both of these harddrives have windows xp on them any help would be appreciated.

To start you cannot add a hard drive with windows XP already installed. It would have been setup for a different computer and the first time it logged into MS server to update it would be deactivated. Second the drivers for the motherboard would not be installed (unless the computer you set XP up with was the same).
To install the second hard drive you only have to jumper the drive as slave. It should have instruction on the drive it's self. Then install it into the dive bay underneth the current drive. Follow the grey flat cable attached to the first drive to the second unused plug and plug it into the back of drive two. It will have a red solid or dottied red strip. Make sure it is closest to the power plug. Plug in an available power plug.
When you boot your computer it should now be the next available drive letter.
Go to My Computer, right click on the drive and use the format option to clean off the drive. You will have added 20 gb to your system.

ok i just did that so what happens now when the one hard drive fills will it start switch over to ther one
or do i have to start saving it to that one as well no matter which one thinks alot

No. When you want to put things on the other drive, you have to actually save them to that drive. If the drive letter is, for instance, E:, then you can put your files in the E: drive. Operating system files and program files should remain on the C: drive, data files such as pictures, music, word documents, etc, can be moved to the second drive.

It is not different than using a CD-R, floppy, or USB Storage. You place hat you want in that location. The thing you cannot replace, pictures, music, work, should be on a drive seperate from your Windows and programs.

The only way that you would have them go to the second drive automatically would be with a RAID and you would have to have SATA drives for this.

Don't forget, if you can't see the drive yet, you need to go to Contol Panel>Administrative Tools>Computer Management, then in the window, select Disk Management from the tree. You should see the new drive there, and you may require formatting it...but that's only if you couldn't see it when you first plugged it in.