Problems with readonly attribute

I'm not sure if it's the UAC, defragmenter, or even Norton 2009, but I'm having certain drives randomly going to readonly and not allowing me to write or read from them. Specifically, it generally becomes noticeable when trying to open Steam. Steam will say something along the lines of cannot write to this drive or something.

I go into command prompt, diskpart, select my volume, then run an att vol clear readonly command, and things are working again, but the problem is a little bit disconcerting. It only seems to happen after I boot up, not in the middle of my work or something. I'm not sure what's causing it, but I thought it was a pattern after defragments, which run on wednesdays. But today, it happened on a Saturday.

Does anyone know what's going wrong here? It also happens to logical drives.

Obviously my UAC is on, I'm on Vista 64, and I'm using a WD Caviar Black 640 GB hard drive.

Erps, found the culprit to the inability run as an administrator and the change my environment variables and configure advanced user profile properties problem... I think i installed a winrar of the wrong version. The readonly problem predates this significantly though.