Windows XP

Dimension 3100

I called dell a couple weeks ago about a computer problem. They said i needed a new harddrive. So today I went and bought one. Put it in my computer. Turned my computer on. Pressed f12 so i got the boot device menu. Then I went to get my moms laptop's windows discs. Put them in, select cd-rom option thing so it would load the disc, and all i got was...

Boot device not available

Press f1 to continue or f2 to retry.

Old hard drive -

Seagate Barracuda 7200.7

SATA

80gb

7200 rpm

New hard drive -

Seagate Barracuda 7200.11 (i think its 11)

SATA

500gb

7200 rpm

Please help. I tried to call dell but my cell phone keeps cutting out before they start helping.