How is your 'Thrash can' configured? You might throw files away after which Windows should report that more space has freed up. But that doesn't always work as smoothly as you would expect (by now). As suggested earlier, using the disk cleanup tools built into Windows fixes this.
Also, over time you Windows installation gets bogged down with applied patches and updates. Windows keeps a spare copy of each, so you can then roll back/forward your system. There are ways to get rid of those files, but do this with caution. Depending on the amount of patches/updates you gave collected over time, you could reclaim several GBytes on your C:\ partition. See this link
for more info.
How is your 'pagefile.sys' configured? Depending on how that is configured, you could reclaim several GBytes on your C:\ partition. By default this is set to let the operating system handle it automatically. Over time the size of this file could have grown to a size that is unnecessary. You can disable the automatic management and set the minimal and maximum size of this file to the same size. I always use a size of 2 times the available RAM when there is 4 GByte of RAM or less. If there is more than 4 GByte of RAM in the laptop, I set the min./max. values to the same amount of available RAM.
Windows 7 defrags your HDD drive on a weekly basis (by default). Once in a while you could use a different tool to defrag your drive. Disk Defraggler (freeware) can also defrag empty space on your drive. Often this results in a little bit more free space.
Do you use the 'Restore points' feature of Windows? By default this is enabled. Depending on how many of these restore points have been generated over time, you could reclaim a GByte or two on your C:\ partition. If you feel competent enough you could disable the feature and remove all remaining restore points. If not, you can reduce the amount of restore points manually or configure the feature to do this automatically for you.
There are usually two folders in the root folder of your C:\ partition, called: 'Temp' and 'TMP'. Open these folders, select everything and delete these files. The temporary files which are still ion use will not be deleted, but most of the collected cruft will. Don't expect to reclaim much free space with this though. You could do this on a regular basis, but not too regular as it can negatively affect your whole computing experience. Myself, I don't care about the results of emptying these folders, but people consider me weird over here.
Those are the immediate tricks that come to mind for reclaiming space on your drive.