>I need to convert an hexadecimal(16) number in decimal(10). How ?
First of all, i think you mean hex(10) = dec(16)
what you can do is something like this

(VFP code, but it is for the idea)
decimal = 0

for i = 1 to len(hexstring)
   lchexnumber = substr(hexstring,i,1)
   lnhexnumber = at(lcHexNumber,'0123456789ABCDEF') - 1 && or another variant of this
   decimal = decimal * 16 + lnhexnumber
endfor