[what do i hav to do to go to the maturity rank. I want to acces a adult location.]

Well, first you have to go to Rausch, there you kill some boars and collect their snouts. Once you have 10 boar snouts, you'll be directed to the next sim and do some similar tasks there.

After a few days of grinding, you should be high enough rank to get into the Adult sims.

[About to change Covenant]

To me, it actually sounds like the estate owner decided to suddenly introduce some rules to give him a seemingly legit reason to boot you from the place prematurely, so he can rent it out to some other poor sod again, rinse and repeat. The covenant bit is just a facade in case of complaints. Most of the people doing this kind of stuff don't even bother with it, but apparently you encountered one of the few who does..

[cannot login at all any more - error message keeps popping up - help pleeease !]

Alternatively, try to log in in an entirely different sim from your home or where you usually log out.

Just pick a random location that you never visited from the website and follow the SLURL, sometimes this works.

[Will a laptop with this spec run sl?]

It's a more than decent laptop for running SL, just keep check on the heat build up. Laptops may be called laptops, but for running SL, you don't want your legs clogging the air circulation from placing it on your lap. It's something to keep in mind for any laptop, no matter how much money you throw at it.

[Get Rid of Default "Hello, Avatar!" Message?]

It's a sort of easter egg. Well actually it isn't an easter egg at all, it's a reference. Back in the 80's, about every single book on how to learn programming started with a sample code to print "Hello World" on screen. The "Hello Avatar" code is just like that: first steps into programming, while also including the most basic parts of an LSL script, to alter as you see fit: a state_default state, and two events: state_entry and on_touch.

 

I think this default script is generated server-side, so I don't think itcan be altered in the standard SL viewer. A third party viewer however could probably be coded to immediately replace that code. I'm not sure whether it'd be allowed to do so according to the license agreement though, but I bet it is possible.