Well, I wanted to make a couple of doom instrumentals which is entirely made on the computer. I have Beatcraft/DFH for Drums, and iPhone Garageband (smart strings) for strings (orchestra). Using what software would I be able to synthesize guitars? I have tried garageband for iphone but it is very limited and incomplete for metal (so is the drumkit but I have DFH anyway). I don't want a software using which I have to plug a guitar in, but a software that will entirely help me generate complete riffs for tracks of varying moods.

I haven't done this in several years, so my info is a bit outdated. For distorted guitars, you definitely need a virtual amp (I used Amplitube Metal, no idea what's out there these days).

For power chords, I had my own Reaktor setup, but it was fairly basic. Several sawtooth oscillators (three or four, I think), slightly detuned against each other and in different octaves. For the palm mutes, you need to use lowpass filters with a fitting envelope. There are different types of lowpass filters, I remember one being better suited than the others, can't remember which though.
This actually worked surprisingly well, though I would beef it up with a clean, low, rich synthesizer in the final mix (not too loud, of course!).

For leads, I used FL Studio's "Slayer" guitar synth. Again, surprisingly good results. You can also use this for power chords if the above sounds too complicated, but I got better results with my method. You could of course also try experimening with simpler soft synths than Reaktor.

Here's the best I've come up with. Can't get any leads, but the low chunky stuff sounds alright. (need Tabledit and Wavepad sound editor for this, both free downloads)

1) Open tabledit
2) Set up a harpsichord, tuned to A#(-2)
3) Tab out whatever you want
4) Record it with Windows Movie Maker's narrate timeline option
5) Open the audio with wavepad
6) Amplify it so the spikes hit the edge of the window
7) Apply a chorus effect (10ms delay, 500hz modulation, depth: 4, gain:100)
8) Apply a phaser effect (2ms delay, 100% gain)
9) Go to directx effects and choose distortion (settings from top to bottom are -12db, 25, 3675, 574, 1206)

Note: If you can somehow get the 3rd setting in step 9 to 8000 you can get an awesome sounding tone...My version might be broken or something, I've only gotten it to do so once.