You are not overreacting. You should feel alarm and suspicious by him deleting his internet history on a daily basis. He might be flirting with women through instant messages. He might have an addiction to going on pornographic websites. He might be going on personal sites like Craigslist by putting up postings and answering women's postings. You are not being paranoid. Your instincts may be right. In my experiences, my last girlfriend would have instant message conversations with guys when I was not with her. She saved them on her laptop. A few of them I found and read. On her home office computer, she kept pictures of past boyfriends and journal entries over the despair of her boyfriend before me who hurt her.
She kept her boyfriend before me on her Myspace as a contact.
When I told her that having him on her Myspace bothered me, she took him off. Weeks later when I went back on her myspace, she added him back onto her Myspace. When I became so upset (I broke up with her for a while), all she said was "He was there all along." I have no doubt that she spoke with him on the phone when I was not with her.
Three or four months into my relationship with her, I found out she was going on a personals website, OkCupid, to find a boyfriend. When I confronted her about going on that website, she immediately said, "Are you stalking me?"
She later told me that she was going on that site to just "fill out surveys."
Like Myspace and other social networking sites, OkCupid, showed the dates that she logged onto that site.
When she dumped me after eight months of being together on my old cell phone, she later went back to going on that website for a while answering more questions (some of which were bizarre).
You have to come up with a way to find out what he's doing on his computer. You might consider getting a camcorder and secretly placing it in your home where the computer is located. There has to be a reason why you do not trust him as he has done something or some things to warrant you not trusting him or guys in your past have hurt you.
I am sorry that your husband's actions are troubling you.
I hope that this helps. ~ Alan