All of the previous answers are good, so I won't try and cover the same ground. Rather, a few things i'd do before gathering further data on performance. These tips assume that the source of the issue is the disk I/O subsystem, which in my experience is generally responsible for the most obvious slowdowns, even if indirectly.
Firstly - clean your machine up. By that, I mean remove any apps that you don't actually use, empty %TEMP% and the various other temp folders your applications might use (including browser caches and other application temporary files), delete all but the most recent system restore point etc. Running Windows disk cleanup can sometimes find further files which you might overlook during a manual tidy up. I actually automate a bit of this with a script which deletes files from various temp folders on boot, i'll leave that up to you.
Next, defrag. Use a defragger like the free edition of Ultimate Defrag which can move files you commonly access to the faster parts of the disk. By default, it moves files which Windows uses on startup to the faster portion of the disk, but you can also tell it to move user files - for example, I ensure my Outlook PST files are contiguous and on the faster part of the disk.
Finally, if you're able to, move your pagefile to it's own partition. This will ensure it doesn't get fragmented, resulting in a performance improvement in paging operations.
Doing this will eliminate "wear and tear" from your investigations by eliminating some of the cruft that Windows installs amass over time, allowing your efforts to focus on specific performance problems.