microsoft.public.windowsxp.security_admin

Can someone please help me with what is probably a simple thing to resolve.
I bought a USB 1GB storage memory stick from a UK well known pc place for me
to add music to it to place in my car stereo's USB port. Everything was ok
and i added loads of tracks but a few days later whe i went to add more i
keep getting error message saying disc is write protected, remove protection
and try again. I cant even delete tracks off it and it came up about me
re-fromatting it but i still get same message. I havent installed anything on
my pc so i know it is not that and i have tried it in 3 other computers and
get same message.
How can a memory stick one minute be ok and the next not?
Its not about money, it was only £8, but if i cant get it sorted i will go
out and buy another bt then if that does same ill out of pocket again. I need
to know how to fix it.
I am using windows XP home edition with P4 processor 256 ram 80gig hard
drive not that that should matter but i dont know what other info you may
need.
Any help will be gratefully received.