Download and use Ccleaner, but ask us before you use the first time for correct settings.
http://www.piriform.com/
I will gladly share my settings with you.
Use this second.
TFC - Temp File Cleaner by OldTimer - Geeks to Go Forums
But post if something you do not understand.
Dont delete anything you do not understand.
For the remaining, from your sister, you have to look at them to be sure they are not wanted. If you are sure you do not want your Sisters stuff just delete. If they are in documents, they are not necessary.
In addition
http://www.vistax64.com/tutorials/76073-disk-cleanup.html
Follow these rules for a computer that is new (at least for you)
To maintain your computer
10 Tips on How to Properly Maintain Your Computer
Steps that I did not notice on the guide include:
Running a scan disc before doing a defrag
Using Microsoft Security Essentials and Malwarebytes.
Making System Restore points, at least once a week
Makaing a system Image once a month (varies by user)
Not relying on your A/V common sense, should prevail. You know of many sites can cause a problem.
Do not use any reg cleaning or optimizing software.
Download and use Ccleaner about once every two months. Untick any registry entries.
http://www.piriform.com/
If you follow these instructions, you will be trouble free for a long time to come.
Be sure to back up your hard drive about once a month (varies by user) and to keeep system restore points up to date.
Optimize Windows 7 also for Vista