Matt Woodward posted about his problems installing Subclipse on Vista. His post includes the resolution to the problem, so check it out. By the way, this is a problem with Eclipse that would effect installing any plug-in that properly declares its size in its feature.xml.
This problem has come up before on other versions of Eclipse. Apparently Eclipse includes some native code to determine the amount of free disk space. When the native code does not work right, or is not implemented, then the Update Manager thinks there is not enough free space to install the plug-ins and does not enable the Finish button on the wizard. I remember this problem happening on OS X with Eclipse 3.1. I thought they (Eclipse developers) were going to make the Java side of the code handle this better, I guess they did not.
Anyway, I am sure a lot of Subclipse users will be moving to Vista pretty soon, so hopefully Matt's post will help them get past this problem. It also looks like if I change the stated size of Subclipse to zero bytes for the next release, that will also avoid the problem. I have added issue 594 to the Subclipse issue tracker so that I do not forget to do this for the next release.
Thanks for posting Matt!