Having a computer problem when signing on.?

My computer takes forever to boot up, even though I've deleted a lot of unnecessary files and other things I no longer need. I have Windows Vista Home Edition, and am plagued by a black screen that appears after I log on, and the "welcome" sign comes on. Formerly, I would then get the desk top. But in the past couple weeks, a black screen comes on after the welcome, and it lasts about a minute and a half before the desktop appears. Apparently, this has happened to a number of Vista users. Does anyone have a solution for that?