I did a little checking on my own computer.
I'm using Windows 98; I have a fixed swap file of 300M, located right at the top of the hard drive (it starts in cluster 3).
I booted to DOS, ran Norton Diskedit and used it to look at the c:\directory and the contents of win386.swp, taking note of the location of the swap file's directory entry and also fo several sectors which contained identifiable text. A couple, located near the begining of the swap file, contained what looked like a list of names of various system calls; a whole bunch in cluster 2136 contained text pertaining to ATT Connection Manager.
Then I used eraserd to do a 4-pass wipe of win386.swp.
Then, while still in DOS, I ran Norton Diskedit again and looked at the c:\ directory- the former entry for win386.spw was gone, with the filename set to random letters and the parameters for starting sector, length, etc. set to zero.
I then looked at the sectors whose locations I had noted previously. The text I had seen was gone, replaced with what looked like random data.
Then I ran Windows and immediately launched Norton Diskedit once again (if you try to run it from Windows, it runs in a DOS shell window). Now ther was a directory entry for a newly-created win386.swp. Looking again at the sectors I had examined before, I found that the stuff near the top of the file was back, but the stuff I had seen farther into the file was still random data.
It looks like eraserd is doing an efficient job of erasing win386.swp, that Windows creates a new swap file when it runs, and that the only data that's common to both the erased and new swap files immediately after launching Windows is stuff that it has to load while it boots, and then shoves off to the swap file.
As you continue to work in Windows, more stuff will get paged off. If you run the same programs and open the same files, it's likely that you can come up with a swap file containing contents similar to the one previously erased- if I log on to ATT worldnet, I'll wind up with the same text strings in cluster 2136 every time- but having the old swap file mysteriously resurrect itself after being erased doesn't seem likely.