Unfortunately, Aricept and it's brethren (Exelon, Reminyl) do not seem to improve paranoia or other behavioral or psychotic symptoms like hallucinations or delusions (like paranoia). The first thing that I do is to be sure the symptom does not represent an acute medical illness. For example, new onset paranoia can be the only symptom of a urinary tract infection in an elder with dementia. I also assess for pain or other aspects of suffering that might present as paranoia. If your mom is in early stages, she might very well tell you if she has pain, but in later stage disease, past where a person can tell you where or even that they hurt, people sometimes just act different when in pain. A change in environment (caregiver, food, daily routines etc etc) can also result in mental changes like paranoia. Some medications (Sinamet is a classic example) have psychosis as a potential side effect. Sometimes the paranoia has a grain of truth to it - there WERE visitors last night, family not recognized because the lights were low so must have been intruders. So a review of what's been going on in her world might turn up a clue and a good look and her medications is a must. Fix the fixable things that are found.
I usually try an SSRI (Celexa, Zoloft, Lexapro, several others) first if the situation is neither urgent not severe. Depakote sometimes works as well. Sadly, these often do not work and an antipsychotic medication (Risperdal, Zyprexa, Abilify, many others) is sometimes used to protect the safety and quality of life of the sufferer. These medicines are not completely safe and should be prescribed by someone with training and experience in their use. they should be used for as short a time as possible and recipients should be monitored closely for side effects.
Hope this helps.