How you do this depends on how the Photoshop file is set up.
If you have your logo on its own layer(s) with a separate background layer, all you need to do is turn off or delete the background layer as the first step. See below for the last step.
If, on the other hand, you have only a flat file with the logo and the background on a single layer, follow these steps:
If there is no white in the logo, the simplest way to isolate it is to use Select > Color Range with your foreground color set to white. That will select the entire background. Reverse the selection (Ctrl-Shift-I or Cmd-Shift-I depending on your platform). If the logo also contains white elements as part of the logo, then switch to Quickmask (just press Q) and with a white, hard-edge brush paint out the red overlay on the parts that you need to keep, then continue.
Click “Refine Edge” (which will show up on the Control bar if you have any selection tool active — click on the Lasso tool or Marquee tool if necessary) and fine tune the selection so you have exactly the logo and no background. For a logo, I would set the feather to about .5 pixels. You should check “Decontaminate Colors” before accepting the changes in the dialog.
This will place the logo by itself on a new layer with a layer mask. You should Save As with a new filename (so as not to overwrite your original file).
LAST STEP: Having got this far using either of the above methods, you'll want to compact everything onto a single, non-background layer. Select all the layers that comprise the logo itself and press Ctrl-E (Cmd-E on Mac) to make a single layer with a transparent background. (Note: Don't use Layer > Flatten, because that will make a background layer with a white background!)
Save what you now have as a PNG or TIFF for use in Powerpoint. You could save as an EPS, but that's a dying format and isn't recommended for most purposes.
Hope that helps.
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