Careful, ppa repositories are experimental and not recommended for newer users. Any beta Kubuntu's are also not recommended. Kde 4.3 beta is likely to break. If you know what you are doing, it's fine.
There's nothing wrong with Kde 4.2.4 (or lower); everything works, including amarok. Just make sure you have all the codecs installed.

Go to your home ie., ~/ and show hidden files.
Then rename ~/.kde to ~/.kde to ~/.kde_old
Restart.
You will lose all your personalization and redo all your personalization with plasma etc.
If you have kmail, konqueror settings, copy from ~/.kde_old these settings.
If you find this hard, suggest you reinstall, but this time don't touch the ppa and beta's.
Reinstall is much easier than windows reinstall. Also backup your home for your data.
I personally find kde 4.2.2 is good enough, but you can go to kde 4.2.4 without problems.
BTW, I am on Karmic now so I can't help you with any specific Jaunty issue. The above is
good for any edition or Linux distro. Good luck.

No, u dun reinstall anything.
Just rename .kde or delete it.
Restart. that's all.

Restart in kde.
Not gnome.
If still break, reinstall.
Dun touch ppa or beta's.
It's for testing and development only, not for normal use.

Yes. this things very often happens for beta testing.
Some dependencies are required, but will break some current apps if installed.
Some testers will force-install (NOT recommended for you) to check.
In this case wait a couple of days to let the testers sort out new libs and this 'error' will go away when the newer libs are ready. (for you, do nothing).
Note: there are easier, but less cutting-edge, linux distro's like PC-Linux, Mandriva, Linux Mint to set you up (older kernels, kde 3.5). Ubuntu is actually quite easy if you don't go cutting-edge, and quite cutting-edge if you can handle it. (it is not a McClaren or a Kanchil, it is both)

Reinstall

Reinstall Jaunty

Ok, your choice, but remember some kde apps like amarok, kaffeine will break when you're on kde 4.3 beta when they should be running on kde 4.2. The kde 4.3 amarok and kaffeine rocks(available on Karmic). Also suggest you uninstall fully(purge) all kde and make sure you got all Gnome apps, like mplayer, braserio etc.

Kde is undergoing a total change starting with kde4 and is quite aggressive in pushing out newer things. Has made many people upset when kde4.0 is not quite ready when it should be called beta. Many distros are not packing with kde 4 and still using kde 3.5.
IMO, I think kde 4.2.2 is the first of the ready version for normal use. Kubuntu will use the lastest that kde thinks it's ready so you have to be careful when using kde on Ubuntu.
As u found out kde4.3 beta is a mess in Jaunty, kde 4.1 is a mess in Intrepid but u have to give kde credit to make a complete overhaul. Everything from sound, graphics to engine is totally different. Gnome is more gradual and incremental but u urself have to decide what u want. Personally, I think Kubuntu rocks but if I have to recommend to a new Linux user, I'll start him off with PC-Linux. He will enjoy it and will most likely end up with Kubuntu by himself and will wonder why he didn't switch earlier from MS.

That would be unusual if you are on Jaunty, but very common if you are on Karmic.
If on Jaunty, the repo's are probably being updated itself, give it a day or two and try again. Check if you have the right 'location' for your updates. Set to malaysia, singapore or nearby or is it 'main' where downloads are slower? If on Karmic, good luck, too messy to write here. you can try "sudo dpkg -reconfigure" first.

For less error prone, use synaptic, go to sources list and change to malaysia or singapore. Also as I said earlier, try again, the repo's may be updating to the newer plasma at the time you are installing.