I was going to answer this, then read it again and now I understand I don't understand the question. Let me see if I understand this correctly. You want to using line input using a guitar amp Plug in? I am assuming a real guitar going to the input?
For the most part, anything going through the audio chain of the onboard sound you won't like. Pretty sure. You would probably be best served to get some USB/Firewire (if you laptop has 1394 connection) and work from there. Just a simple two channel and usually in mono (adding stereo out later for mix down).
You can usually get a decent USB interface for around $150 or less. So, do yourself a favor, start looking into something like this so you start out with the best setup to give you positive results. (less frustration when you hear it). It is just a few more dollars more and a whole lot better in the long run. (don't forget headphones, any kind will do first starting out, it will sound better than your laptop speakers)
Not sure I helped here, but wanted to give some input to keep you moving forward in your quest.
I assume emulated output (sorry, I am sure someone else understands this better) is just an out through the head (by passing the speaker) if so, the answer is yes, just watch the input gains. On my setup, when I play electric, I go straight in through a mixer and use the DAW's VSTs for amp simulation. So, in other words, you don't have to mic the amp, but you might loose the room. Just have to play around and see. But you can do that, just really watch the output that you don't peg the input. You might look at a Sure SM58 mic to mic the amp with speaker. Lots of people seem to like doing that for the sound. A lot of folks here like the 58's as a general all purpose mic.
It depends on what you mean "good enough for home use." Line inputs on laptops (which are not really line inputs as they have some preamp capability for handling mikes) are of relatively low quality compared to even a very inexpensive standalone audio adapter. I use my lap top for recording when I work at my writing partner's house and I would never consider using the line input. I have an M-Audio Fast Track Pro which has both balanced and unbalanced line inputs, and has the advantage of accommodating microphones that need phantom power. I'm not saying that this is necessarily the device for you, and there may be reasonable alternatives out there that are cheaper, but you should definitely consider an audio adapter.
FWIW, you won't harm your laptop by plugging it into the amp PROVIDED that the amp is properly grounded and, if the laptop uses a grounded power supply (many do), that is grounded, too, and both are plugged in to the same leg of the AC power. Start with the volume turned to minimum and SLOWLY bring it up by watching whatever your laptop uses as an input monitor (it's also possible that the line out on the amp is fixed volume, in which case the amp volume control won't affect anything, but better safe than sorry).
I think you see a pattern here. As stated above, this is probably your best option that will yield you a pretty good result. There are some threads that talk abut latency and so forth, that would be the primary concern as you will want to probably record and play back at the same time. But you can ask about that in another thread and get even more opinions and references.
Is it possible to get a very basic USB interface just for a single input ( FROM GUITAR AMP ) so I record the tube amp overdrive
The M-Audio interfaces have mike inputs which I don't need and they look like you connect the guitar into it, what about drive and effects if you guitar into the interface direct ?
The M-Audio interfaces (and most others) have line inputs as well -- they're dual jacks, i.e. you can plug in either a balanced XLR or 1/4" RCA. As for drive and effects, you'd plug your guitar into the effects chain and the effects chain into the interface.
I think OP is trying to find something similar to iRig or Apogee Jam. Though as I believe these are both for Mac, perhaps someone knows of a Windows version for OP to use?