Hello,
first of all, i have got here a very old FUJITSU SIEMENS SCENIC E600 i865G computer with XP HOME EDITION as OS.

My problem is when i turn on the pc, it happens FUJITSU SIEMENS logo appears.
But after that a black screen with a STRANGE loading white bar (made with white vertical lines) below appears.
Something like this:
||||||||||||||||||||||||||||||||||||||||||||||||||
that loads until last line.

After that, windows xp home finally loads but very very slowly.

My questions are:
1) what's the real matter with my pc ?
2) how can i solve its damage ?

I think you are going to need to get someone knowledgeable to physically look at it. there are too many things that could be an issue.

1) XP - this is a problem. You are not getting updates and XP is ripe for getting malware, hacked.
2) you could have filled the hard disk
3) could have not enough ram
4) could have loaded too many programs that start when it loads

there is so little info to go on, it is really not the right medium to trouble shoot it like this.

first of all, if you need your data, you should connect HDD to another PC and make backup of important files.
Then you can check all components of your PC - PSU, MB, RAM, HDD
May be it's OS problems, and you can re-install it, for example