Can´t write nothing in \bin folder in windows

Hi all,

well, it´s just a windows xp professional file permisions problem, but some other folk had the same problem so...
i´m trying to upgrade my postgresql version ( it was 8.2) in windows , so i' ve downloaded and installed postgresql 8.3.3.1 When i try to install postgis 1.3.3 with the Application Stack Builder and after downloading the postgis installer, appears an error

"Error opening file for writing"

everytime it wants to copy a file to the install_folder.

It seems a problem in folder permissions. Windows has a strange behaviour with some folders. They appear as read only and you can´t change that, but it´s not a problem when you want to copy something into it or delete it. For example, i can copy into postgreSQL 8.2 folder and delete this folder. But when i install 8.3 i can´t copy anything into its \bin folder ( for example) and i can delete all folders.

i can´t found any info in Inet, so perhaps there is a problem with my computer... Any examples of successfull installation will be welcomed ! :)

maybe something has changed in postgresql installation method?

Thanks in advance!

Gus