When I install any software, that comes with installation package in OS 10.3.8, these installation packages are copied into Receipt folder.
I wonder, could I delete them from my HD ?

Mac Hard Drive -> Library -> Receipts -> Installation Package (ABCD.pkg).

The other things inside this folder are installation text logs which I'm sure can be deleted.

Don't touch them, you need them to correctly repair permissions, which is something to do each time you install a program on you mac.

if you think you need to trash some logs, use OnyX, you'll have beside the possibility to repair permissions, optimize the system (pre-binding), and run the BSD maintenance scripts which are made to run when people sometimes have not their computer running. You can clear a lot of system files than can be bothersome or useless if needed.

One IMPORTANT thing in OS X as in all Unix OS, if you don't know what does something do, don't touch it!

You need these packages, capice?

Thank you very much.