Sikkerhedshul på hjemmeside.

Hejsa.

Jeg fik en besked før i dag, fra en af mine venner, at han ikke kunne se gæstebogen på min hjemmeside, da hans antivirus gik amok.
Jeg fandt så ud af at der var tilføjet noget javascript i en af filerne. Efter at have fjernet det, virker det hele fint igen. Min hjemmeside ligger hos UnoEuro.
1. Hvordan kan dette script pludseligt dukke op i min fil? Det er kun mig der har FTP adgang til den.
2. Hvordan sikrer jeg mig at det ikke sker igen?
3. Hvad gør dette script? (Se scriptet nedenunder)

Her er scriptet:
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