Hi,
can anyone give me an idea about how does the quality of scaling and deinterlacing of a Video Processor compare with that of an HTPC ? ofcourse assuming a good quality GPU and/or player software.
Is it comparable ? or better ? or worse ?

PCs are capable of MUCH better than ANY video processor, including super expensive commercial ones used for authoring DVDs. It's more a question of:

-Ease of use
-bootup time
-Efficiency - power draw
-Reliability - what will you do when the power cuts out and screws up data on your hard drive? Or what about when the hard drive dies?

And even with the power of modern CPUs, the best of the best filters still often can't be used in real-time. You can get something comparable to consumer video processors for cheap from a computer (it doesn't have to be super powerful), but all of those drawbacks above still apply.