The following describes MicroSoft's recommendation of a very clumsy way
of doing it (for Win95 and Win98, but I imagine it'll work in XP too):
http://tinyurl.com/6utc
You might be better off getting a more full-featured sound recording
app. A quick look found something called PolderbitS Sound Recorder,
which seems as if it would do just what you want, but I've never used it
and have no opinion about it one way or the other. Take a look:
http://tinyurl.com/mn1m
If you stay with Sound Recorder ---
because you are recording spoken audio, you don't need CD-quality
sound. Note that at high quality, a wav file is about 170KB/sec -- thus
an hour of that would be 612,000 KB. To get a more manageable file
size, go to the File menu in Sound Recorder and click Properties, then
"Convert Now" and select one of the lower-fidelity formats. And as you
would expect, mono takes up less space than stereo. Unless your tape
was recorded with two mikes, mono is all you need.