I fully agree with the screen size - in a session, you may not only want to have the Karaoke software, but maybe also Firefox or a search tool or a text ticker tool... a lot of things which fits much better on a larger screen. I looked at the other offers from BestBuy. If I would have to make the decision, I would add $50 and buy this one:
http://www.bestbuy.com/site/olspage.jsp ... 6470439579
Same technical data like your first choice, same brand, but 17.1" screen.
And for Windows Vista: There is a way to downgrade from Vista to XP - but it must be written in the license (EULA) of Vista. Best you ask your dealer.
For the Software: This depends very much how much comfort you want, which features you think you need and which type of music you want to use (CD&G, DVDs, MIDI/KAR,...). There is not "the one and only" solution for everybody.
You may take a first look into Karafun, which can be downloaded for free:
http://www.karafun.com/ I think it is a good starting package, allows many different file formats, is very simple to handle and do not offer confusing features. If you search here, you may find some more threads regarding the right software which may help you with your decision.
For music, many people here recommend the 100 $ package for 100 songs from Tricerasoft (
http://www.tricerasoft.com ), which allows you to download exactly the songs you want. (CD/DVDs may be cheaper, but from 20 songs you may only find 2 or 3 useful).I like the possibility to even be able to download during a session (if Internet is available) - so I have for very low price the chance to choose from 10.000 titles - very helpful when you start with limited budget and have no idea what people want in your bar.
As you are also DJ, it may be clear that you will also need a hardware mixer and at least two microphones - so I do not commend this.
Just my 2 cents
Wolfgang