Anyone besides me have trouble installing components in J1.5. I browse, select and upload. Things just STOP there. in some cases, nothing happens. Other times, nothing happens AND when attempting to install again, it tells me it's already there even though it isn't. I've dealt with this with more than one J1.5 site. Never had this problem with J1.0. It's not an issue with permission setting for the components folder either. If anyone has any ideas, please share. Thx.
What Mike suggested to fix I'd try. It's quite a common issue.
Another common problem with Joomla, especially in shared hosting environments, is the host configuration of PHPSuExec. It's a permissions issue many people run into. Here's an article I ran across. Instead of explaining it in technobabble 101, this article explained it so even I could understand.