521 intermittently slow and freezing

I chose this model because everybody raved about how fast it was, but it's been intermittently slow for me from the start. I have not upgraded anything (tried to upgrade the memory, but it wouldn't start up with the new memory), other than installing Office 2007.

Sometimes it only takes 15 seconds to start Word and open a document, but sometimes it takes nearly a minute. When I open a new tab in Explorer, it sometimes takes only 5 seconds to pull it up, other times it takes up to a minute (and it can also take a very long time to switch between tabs, often freezing for 20-30 seconds). It runs faster when the fan is not running. When the fan is running, it slows way down, and will often just freeze up entirely, with the little spinning circle and the program "not responding," which lasts 30-60 seconds before it starts working again. It also takes a really long time to switch between programs sometimes (such as going back and forth between a Word document and Explorer). I've checked the task manager, and can't find any extra programs running, and the cpu usage isn't high.

Would upgrading the memory or Windows (it's still on the Windows 7 starter edition it came with) help? If so, any tips for doing this successfully? Or is there something else going on here?