Making use of your old desktop - Scenario 1

Scenario 1: Using your old desktop as a media, file and download manager.

Why media manager?

If you already have like 5 gigs or more of media (music/movies), it would be an overkill to have another copy on every new system you buy.

Even if you have an external hardrive, you would have to compromise with your newly purchased laptop's mobility since it would always need to be connected to the external hdd whenever you feel like watching a movie or listening to your favorite music.

Having your old desktop serving those media files over home lan network makes more sense.

Why download manager?

If you download a lot of torrents, you'd have to keep your laptop running till all your torrents finish downloading -- again compromising on your laptop's mobility (and probably its life).

Having your old desktop as a download manager makes more sense.

Prerequisites:
Before we go any further, let me mention a few things here
1. Since your old system would be literally running non-stop, you need an operating system that requires minimum maintenance, and is least vulnerable to spyware and viruses.
2. Your old system should also have an ability for remote management. You should be able to control literally everything on your old system remotely from your laptop

The above two points hint at installing Linux on your old system.

Also I assume that you have a home network i.e. a router. The setup would look something like this:

Basic steps:
1. Install Linux on your old desktop (preferably Ubuntu)
2. Install FreeNX remote desktop management utility.
3. Install Jinzora media webserver.

I'll elaborate on these steps later.