Some children can't focus and attend as well as others, but this doesn't mean they have the condition called ADHD (Attention Deficit Hyperactivity Disorder). This condition is difficult to diagnose when children are young, since it's common to see many of the behaviors associated with this condition in young children who are developing normally -- just because they are young.
However, you did describe behaviors that might indicate that your daughter has ADHD. You mentioned that she can concentrate and focus on some days but not on others. This intermittent inattention is often seen in children with ADHD. You also said that she can focus if she's interested in something, and some kids with ADHD can do that, too. The novelty or high interest quotient of certain activities (like TV or Nintendo) can captivate kids, and at those times, it's hard to believe there's anything wrong. Your comment that she's very distractible at home and at school means that this problem affects her in two different settings -- an observation that adds fuel to the ADHD hypothesis. You also say that she doesn't complete many tasks or that it takes an unreasonable amount of time to complete certain tasks. This too may be an indication of an attentional problem.
Before we rush to put a label on your daughter, let's consider other things that can make it difficult for a child to focus or attend. Children with hearing problems obviously miss a lot that goes on in their environments, so it's very important to make sure your daughter can hear her teachers and classmates. A hearing test can answer this question. Kids who aren't eating right, who don't get enough (or the right kind of) sleep, or who have allergies often exhibit some of these symptoms, too, so make sure you and your daughter's pediatrician rule out these causes. Other children are worried or anxious, and this makes it difficult for them to attend consistently. And remember, kids can't always tell you what they are worried about. A consultation with a child psychologist might be necessary to sort this out.
You said that you thought your daughter's behavior might be a way of avoiding certain tasks. This is certainly a possibilty, but we need to ask why she would want to avoid tasks in first or second grade -- stuff that most kids think is pretty darned fun. Some very bright children quickly lose interest in topics they have mastered or they think are boring. Other children "tune out" when they think the material is too hard for them, or if they have learning disabilities (LD), a condition which makes it difficult to process information they see or hear, even though they are quite intelligent.
Since your daughter's problems are getting in the way of her school work, you should request the school to call a meeting of the child-study team (it may be called something else in your child's school) to determine whether everything that can be done in the regular class is being done. If teachers say your daughter is still having problems despite their best efforts, then your little girl should be referred for an evaluation by the special education department. Testing and observations, plus a consultation with your daughter's pediatrician should help you determine whether your child's difficulties are due to LD, ADHD, or something else.