Hi Ekhubbar,
First of all, if your laptop is still under warranty and suffering from a hardware fault, it may be worth checking with Toshiba in the first instance so see if they can replace it. If you can provide any details about why you require a new hard drive, I may be able to assist further, otherwise if you are just looking for a replacement of the same size and spec, see below.
The product spec of your system quote that the M45-S165 comes with :
"80GB (5400 RPM) Enhanced IDE (ATA-6) hard disk drive; 8.0mm height; user removable"
Doing a quick search reveals a price at around $60.00 (for a drive made by Toshiba). However if you are going for something cheap and cheerful from egg, you may want to check out the $35 drive. It is the same size in storage, and spec wise it will access your data faster.
Here is a link to the specific product in question (on newegg.com):
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16822144102So it would be cheap to buy a seperate laptop hard drive, but do make sure that you have a professional replace it for you.
Hope this helps,
David