Having serious Itunes problem, please I need your very urgent help!!!

Hi to all.
I am having serious problems with Itunes.
Before I write about it, I must say I am on Windows XP and I have the latest version of Itunes installed. I've tried reinstallation and complete remove from registry a few times, but it did not help at all.
Wwel, at first, when I plug my Iphone to computer I get something like the following: "It cannot be used because Apple Mobile Device service doesn't start automatically." Although I went to control panel/administrative tools/services/ apple mobile service and had it set to start automatically, I had to click on "run button" and after that Iphone was recognized until I disconnected and plugged it again so I get the same error mentioned above Every time after pluging it back to the computer.
When, by any chance, I make Itunes to recognize my Iphone and when I want to sync apps, music and other things, I check the required checkboxes and press sync button. Although it seems the phone is syncing, it doesn't do absolutely anything. It says something like: "Backing up - syncing 1 step of 4.", but it is a kind of stuck and it stays like that till I hit "cancel button", but when I do that it asks if I am sure about that I want to quit the program because the iphone is being synced. I am confused because when I click "no" I have tried and let it syncing for about an hour and nothing has happened, and I had the same on the screen.
The last thing is I cannot access App store with Itunes. After I run Itunes the following messages keeps popping up: "Accessing Itunes store" and there is only "stop button". It pops up every ten seconds or so. After a while I can sign to my apple account, but when I am about to view account to edit settings and other information, I can put my apple Id and password, but I get the same error and cannot change anything. It also doesn't let me look up the things in app store.
I've tried accessing app store from the Iphone and it works perfectly, but there are some things I'd rather do or Access through Itunes.
Not-forget to mention, I have tried itunes on more computers with different types of connection and at different places but just my home desktop gives me that problem.
I’ve read about all this in a few Apple discussions where they have recommended fixing the problem by unauthorizing and authorizing again the computer. I’ve tried that but without any success.
The problems have occured a few days ago; before all that happened things were fine.
I am very sorry for that long post.
If there is anyone who could help me solving the problems, your help would be very greatly appreciated.
Thank you very much in advance.
Feel free to contact me at the following contacts:
Twitter: maya1983416
E-mail: maria.orovcikova@gmail.com