cache not working - Permission denied in the folders

HI!

I've got a list of this kind of warnings at the top of all the pages:
Warning: fopen(D:\Inetpub\webs\mysitecom\opencart/cache/cache.product.latest.2.8.1247074347) [function.fopen]: failed to open stream: Permission denied in D:\Inetpub\webs\mysitecom\opencart\system\library\cache.php on line 39

If I set error_reporting(0); in the startup.php obviously I don't get any error message anymore but anyway I should solve the problem. Cache doesn't work at all.

It seems I don't have right to write in the folders. I also have the same problem at installation stage: it couldn't write anything in the config.php. It said the installation was successful but both the config.php remained empty and i had to write it manually by myself.

In the pre-installation page it's written that all the file and the folders are writeable and checking the permissions on the servers folders are set to 777 and files to 666, so I don't know why I have permission problems.
The server is windows.
P.S.
Register Globals is set to ON
Magic Quotes GPC is set to ON
cURL is OFF
I don't know if any of these could cause this problem.

Thanks for any help.