I want to change my default font in Windows 7 from Calibri to Times Roman?

I assume you are talking about MS Word 2007. As you know it, Word's Normal template uses a new default font, Calibri 11. If you have been using Times New Roman as your default font, you still can — simply change the default.

For example, to change the default font for the document you are working on to Times New Roman 12, follow these steps:

1. Under the Home tab, click the Styles group button to open the Styles window.

2. Click the Manage Styles button, which is third from the left at the bottom of the Styles window.

3. Select the Set Defaults tab.

4. Scroll to and select Times New Roman under Font.

5. Under Size, click 12.

6. Select the Only In This Document check box.

7. Click OK.

Now you no longer have to reformat your Word documents to work in Times New Roman 12.

If you are working on your own machine, you can set the default font for all new documents by clicking the New Documents Based On This Template check box in step 6.

For 2007 & 2010 versions of Word:

1. On the Home tab, click the Font Dialog Box Launcher, and then click the Font tab.

2. Select the options that you want to apply to the default font, such as font style and font size.

3. Click Default, and then click Yes.

http://support.microsoft.com/kb/291291

Edit:

If you are talking about Excel 2003:

1. On the Tools menu, click Options.

2. On the General tab, click a font in the Standard font box.

3. In the Size box, type or click a font size.

4. Click on OK.

For Excel 2007 and 2010 versions:

1. Click the File > Options.

2. In the Popular or General category, under When creating new workbooks, do the following:

3. In the Use this font box, click the font that you want to use.

4. In the Font Size box, enter the font size that you want to use.

5. Click on OK.

Note You must restart Microsoft Office Excel to begin using the new default font and font size. The new default font and font size are used only in new workbooks that you create after you restart Excel. Existing workbooks are not affected.