I have 80 gb hdd, but it shows only 75 gb
pls suggest me what to do to view or enable 5 gb space

That's quite normal, my 80GB HDD shows up as 74.5GB, or 80 billion bytes, it's all in the way manufacturers rip us off with technicalities...

Depends on how the 80GB is advertised. Simetimes its the unformatted size, so once a partition and filesystem is added, some space is 'lost', the size depends on the OS and filesystem.

Sometimes when they say 80GB its either 80GB using binary 1024 or decimal 1000 bytes.

And no, you cannot get the 5GB back.