I have a friend who has a Dell laptop with a bad hard drive model WD400VE.
My question: What other harddrives will fit into this computer?
I know very little about lap tops. Most of my knowledge comes from desktop computers so any info will be appreciated.

Without knowing the specific model number of the laptop I cannot tell you the maximum capacity for a replacement. I can tell you that any standard 2.5 inch IDE laptop drive will work, at least up to 120gb capacity. When you remove the old drive, you may initially think that the old drive uses a different data connector than the new one you have bought. I can assure you that if you look closely you will realize there is an "adapter" attached that will transfer to the new drive. Newegg has drives that will work with this laptop starting at $50 (40gb, 4200RPM) and for ten dollars more at $60 they have a Seagate Momentus ST980815A 80GB 5400 RPM drive. Which is twice the capacity of the original drive.

Sense you mention an adapter on the drive, is there a way that I can hook the laptop drive to my desktop computer? I would like to try this to see if it will work that way.
It gave an error when turned on and then we tried to reload windows and it gave the same error and said there was not a drive present. That is the reason we thought the drive may be bad. She is not home at present but I will find out just what the error message is and report.
The model number of the laptop is PP21L Inspiron 1300.

Most laptops have a disk test in the Bios.