Just recently ran into an issue with extension uploads on Joomla 1.5.x on a Windows Plesk server. The host was Lunarpages.
While there are lots of posts on this issue trying to implicate the php version etc. The issue really came down to permissions plus the tmp and logs dir path. Most host providers will tell you that the tmp dir should be the relative root for the site, however on plesk there must be a complete server dir path for these two settings in the Global Configurations section of the administrator settings:
Log in to the Administrator section of the site.
Click on Global Configuration
Click on the server tab.
Under Path to TMP folder put: “D:\Inetpub\vhosts\mywebsite.com\subdomains\mysubdomain\httpdocs\tmp”
Click on the system tab
Under Path to Log Folder put:”D:\Inetpub\vhosts\mywebsite.com\subdomains\mysubdomain\httpdocs\logs”
Hope this helps others. leave a comment if it does!