If it was just after booting and the RAM usage was at 50% or higher than you might want to look at what's starting that doesn't need to but only 20%? That's not bad at all
I recommend you download CCleaner as well, as it is an excellent program, no so much for freeing up hard drive space, but for fixing registry problems, however it does not allow you to view all the files in your hard drive like you would like. WinDirStat analyzes your hard drives and then makes a chart that shows every file and how much space it uses.