howto Building a remote to control pc with lirc under linux

You can use any remote control have lying around handy with lirc, you just have to let lirc "learn" the codes and then assign them to tasks. This is the easy way to do it.

Now if you still want to build your own transmitter then you need to study up on the various protocols to get a feel for how they work. Then you pick one (or design your own) and go from there. I would use a PIC, but you could do it with more parts and not use one. Usually you have a carrier of about 38kHz that is modulated in some fashion, usually by switching it on and off. RECS80 and RC5 (not to be confused with RC5 encryption techniques) are two popular protocols. This should give you a decent list of things to Google so have at it a bit and come back with more questions. :-)

If i just transmit a pulse of around 38 khz ,would it be taken as a 1 by the receiver? In that case,since lirc can learn any protocol,i can just make up my own protocol :) , i.e. basically permuting a string of 'n' bits,if i want to have 2^n commands at the computer.I only intend to use it in a small room,where there are no other sources of interference. Would such a simple scheme work with lirc?

Pretty much. There are other freqs around 40kHz that some receivers are sensitive to, so know what you've got.

There are lots of schemes for modulating the carrier. On top of that, there are lots of ways to frame the data on the modulated carrier. Even if you want to roll your own hardware, you'll probably get the best results with lirc if you mimic some existing device. I know there's lots of info on Sony remotes if you google it.

What he said. :-)