Im thinking of building a HTPC with a laptop hard disc, will it fit straight in or are the laptop drive physically not as wide and therefore require some kind of adapter ?
you will need to get an adaptor to physically get them to fit.
Regarding the electrical connections, if you are using s-ata then they appear to use the same connectors (much to my surprise).
If you're using IDE then you'll need an IDE adaptor (3.5" to 2.5").
They are, but once it's booted up, and applications loaded they're ok. Low power/low heat/low noise. I'm using a 2.5" IDE 60GB for my desktop HTPC OS drive, with two Samsung 500GB for stoarge.
I believe for SATA laptop drives the power and data cable is the same as 3.5" drives, but with IDE you need a 3.5" to 2.5" adaptor with a molex connector.