Problem with Excel 2007

Hello,

I am experienced using Excel, but I am having some problems and can't find help on forums anywhere on the internet.

I am using Windows 7 Professional on an Acer Aspire 5750 - Intel(R) Core(TM) i7-2670QM CPU @ 2.20GHz, 64 Bit OS.

The problems I am experiencing are:

1. I have one Excel file open, and when I try to open another it appears to attempt to open it but the file does not appear. There are no error messages. Sometimes if I minimize the open file a couple of times the new file eventually opens. And sometimes I have to close the file and the second one will appear. Sometimes after several minutes of just waiting the second file appears, and sometimes it never does, even if I try to open it more than once.

2. When I manage to get both files running together, I switch between them by either using Alt+Tab or using the cursor. I do not use them side by side.
When I flick between them I sometimes, but not always, see a distorted array of cells. This is difficult to explain, but for example, if I want to select cell D1, the information may be appearing in-between Cell C1 and D1, it does not line up properly. I have to minimize the screen a couple of times or save the file and re-open it and the file is back to normal.

3. I was recently editing a large Excel file (3.69MB), there was 39550 rows of data. I had to make 2 new columns and enter data for each row. To save time I filtered it into the 112 categories and selected them one at a time. For the first one I entered 2.8% and 4201.00.6000 in the top 2 cells, highlighted the new data and dragged it down so that each row in that category had 2.8% and 4201.00.6000 in the new columns.
After completing the first category, I found that some rows in the other 111 categories randomly had 2.8% and 4201.00.6000 in the new cells that I had dragged. I had only ticked one of the filtered selections, so there should be no reason for those rows to have appeared when I was dragging the new data. I am now worried, because what if as I was going through the sheet and dragging new data into the cells, it was changing rows from previous selections? I had a quick scan through and it seemed okay,

If you have experienced this before or know why this is happening to my Excel documents please help.
I have no problems with any other software.

Regards,
Justine