Aug. 19, 2008  -- A month after being diagnosed with breast cancer, actress Christina Applegate, 36, is "100 percent" cancer-free, she told "Good Morning America's" Robin Roberts in an exclusive interview.

"I'm clear. Absolutely 100 percent clear and clean," the star of ABC TV's "Samantha Who?" said. "It did not spread -- they got everything out, so I'm definitely not going to die from breast cancer."

But the price she paid for that peace of mind was high.

To be sure the cancer would be completely excised, and that there would be a reduced chance of it returning despite Applegate testing positive for the BRCA1 breast cancer gene, the actress opted to have both her breasts removed in an operation known as a prophylactic double mastectomy, even though cancerous lumps were only found in one breast.

"My decision, after looking at all the treatment plans that were possibilities for me, the only one that seemed the most logical and the one that was going to work for me was to have a bilateral mastectomy," Applegate said.

Three weeks ago, she had the dramatic operation rather than undergoing other longer-term treatments like radiation or chemotherapy.

"I didn't want to go back to the doctors every four months for testing and squishing and everything. I just wanted to kind of get rid of this whole thing for me. This was the choice that I made and it was a tough one."

Though she will be undergoing breast reconstruction surgery over the next eight months, Applegate said the emotional toll has been heavy.

"Sometimes, you know, I cry. And sometimes I scream. And I get really angry. And I get really upset, you know, into wallowing in self-pity sometimes. And I think that it's all part of the healing," she said.

But Applegate is healthy and calm now, due to both her unflappible sense of humor -- "I'm going to have the best boobs in the nursing home" -- and the powerful inspiration she gained from her mother, Nancy Priddy, a repeat breast cancer survivor.

"She's been sort of this quiet warrior in the back and has been a great support, and just telling me that I was going to be OK. And I knew I was going to be OK. I've watched her," she said.

Christina Applegate Fights Breast Cancer with Double Mastectomy

"I've watched her have a mastectomy, and then I've watched her go through two years of chemotherapy and eight surgeries and a hysterectomy. I've watched this woman survive both those things. So, for me, there was always that sense that I was going to be OK, no matter what."

Applegate is also going to be OK due to her own vigilance. She started getting mammograms six years ago, after she turned 30, and said the cancer was found through the second of two MRI tests as a follow-up from a biopsy she had last year.

"If this had been caught a year from now, or when I was 40, I probably wouldn't be able to live through this," Applegate said.

The MRI Applegate underwent, she said, is capable of seeing cells, meaning it can detect cancerous cells even before a tumor forms.

The problem for many other women, though, is that this type of MRI is expensive, and often health insurance does not cover it. So Applegate is starting a foundation to help high-risk women meet the costs.

"It's incredibly expensive," she said. "So, for me, one of the things that when this is all happened, was that I'm putting together a program that's going to raise money to pay for MRIs for women who are at high risk." Applegate defines "high-risk" as anyone who has had breast cancer in their family, or are "gene positive" for the BRCA1 gene that is linked to breast cancer.

"We're also going to pay for the BRCA testing, because that's very, very expensive."

Now, with the cancer behind her and recovery ahead, Applegate is optimistic.

"You know, I really love living and I really love my life. I knew from this moment on, it was only going to be good and that was going to be coming," she said.

"Yea, I'm going to face challenges, but you can't get any darker than where I've been. So, just knowing that in my soul gave me the strength to just say, 'I've got to -- I have to get out there and -- and make this positive."

"And you know, I'm going to have cute boobs till I'm 90."