Hi molliedog.
With regards to your niece doing "something to my computer last time she was down to stop it being so slow".
Is it possible she did a disc clean-up to "free" space on your HDD(Hard Disc Drive) therefore making your pc faster.
If so, go to Start - All Programs - Accessories - System Tools - Disc Cleanup
This will give you a list of types of files to delete.
eg. Recycle Bin, Downloaded Program Files, etc. and click Okay.
If you do this regularly, you are helping maintain your pc.
Hope this helps.
Andy