Upgrade HDD for Presario 5008US

Hi everybody. I'm new to this forum but I'm certainly not new to computers. Let me answer this first: yes, I have done my research, but I just couldn't find enough information to fully answer my question. My problem is this:

I'm trying to upgrade to a 160 GB HDD but my BIOS isn't recognizing more than 128GB. I went the the compaq website and downloaded the newest BIOS update which says: "Added 48-bit LBA boot support to handle drives bigger than 128GB." I had installed the HDD before I installed the BIOS update and the Windows XP seemed to recognize it just fine. I was able to partition the drive into 3rds and read and write to all of the partitions. I was not using it as a boot drive at all, only for more space. When I try to upgrade to Vista (just to try it out), I get to a screen that says something about the BIOS not understanding the HDD partitions and I can't continue. I also have a program called SpinRite that let's me have information to the HDD upon boot-up. When I start my computer with SpinRite, two of the partions show up just fine but the third tells me that I can't access it because my BIOS doesn't understand it.

I went to the compaq website and did an online chat with a technician. He told me that the BIOS update can add access to more than 128GB, but not much more. Misleading? I think so. I would also think that if it could handle more than 128GB it would default my 160GB to that number, but it only defaults to the 128 GB.

My biggest question is this: Since Windows XP seems to recognize the HDD just fine, will I still have access to all 160GB even though the BIOS only recognizes 128GB? Does anybody know how I can get my virtually-useless BIOS to recognize the HDD correctly? I could fill the entire drive and find out, but who has an extra 160GB lying around to find out? I suppose I wouldn't care too much what my BIOS thinks as long as Windows allows me to access the entire 160GB. I don't know if I can even return the HDD now (I've been trying to get it to work for 2 weeks now).

Also, less importantly, when I set my jumper to reset my CMOS and then set it back, after it checks the RAM it says "162 - System Settings Not Found" (which I could understand since I just reset the CMOS). Then it says "CMOS Invalid Checksum - Loading Default Settings". What does this mean? Could this be what's causing the BIOS to not read more than 128GB?

Thanks in advance for any help that anybody can give me. I tried to do my research as best I could (I'm a guy, I don't like asking for directions), but I just couldn't find the answer I was looking for. I apologize if the answer is right there in front of me, but I did try and find it.