I am going to assume by your question you are not exactly computer savvy.
Windows uses a lot of storage space and there can be many factors as to why this can increase drastically.
1. Internet Cache. I assume you browse, and all that goat porn is usually downloaded and stored, at least untill you empty the cache.
You can set your computer to automatically empty this everytime you close the browser.
At modern speeds a cache is defunct anyway, it's orginal purpose was intended for modem speeds.
Ah 14.4kbps, those were the days.
2. Standard email, again, those 3 penis boosting mails a day that make you feel inadequate, or those 4 emails from genuine Russian ladies who make you feel so loved, they all add up.
Empty your mail folders.
3. Windows update, this little bugger does not care if you will never use a function, if there is an update for it, chances are you are getting it.
Windows update does not feel remorse, or pity, and it absolutely will not stop, ever.
You can alter the settings and choose to only install the updates you actually need.
4. Virtual memory, I believe the default setting is to allow windows to manage it automatically, which basically means you are allowing your system to use whatever hard drive space it deems necesarry to run.
You might want to check it out.
5. Malware, chances are if you have any blood pumping at all, you have ocassionally been to some sites that would make a whore blush. Scan for malware/adware on a regular basis.
6. Restore point.
Everytime you change something, Windows will create a restore point, and whilst it is not always a good idea to turn this function off, it does use space.
I generally do a manual restore point prior to installing updates or software.
It seems that if you only actively download a gig in 2 months then you probably have a stable system.
The thing that prevents many of us from just simply reinstalling Windows every time something goes wrong is the amount of data we generally do download. (Music, movies, etc)
You might want to consider just doing a clean install, back up the stuff you need on a pen drive, a DVD or even use online storage.
I do this every 6 months anyway, it's just common sense.