What can I do about this?

I actually think your mother just feels more comfortable with you than you her;; it IS her house & she truly is entitled to go anywhere, check out anything that she pleases, within (her own) reason;; she's simply checking your computer to keep you "safe";; & she doesn't care about you "changing" cuz she (I'm SURE she's told you) used to change your diapers; if YOU don't feel comfortable, the only thing you can do is TALK with her; that doesn't mean 'bark', or 'whine', or 'accuse', or 'badger', it means TALK..good luck, but she's lived your life of you barging in on her, she's just not used to the "privacy' aspect of her home.."all's open in love & war" is where she's probably coming from..again, she just feels very comfortable with you, you just are 'separating' from her...good luck, but talk, listen, compromise...