Start the disk cleanup utility that comes with Windows. Not the best, but an acceptable start.
It will remove much useless temporary files.
I use CCleaner to do my cleaning. I don't know how it compares to Advanced SystemCare proposed by Sean. Both are free.
That alone can accelerate your system.
Next, do a disk defragmentation. Files fragmentation can seriously slow down file access and you have to wait for that.
Maybe your browser have to many plugins and addons.
Take a look at those that are installed and disable or remove those you don't, or no longer, use.
Kualinar
March 2010