That's what I thought.....
Well, you have a few choices. The first is to try to remove items from your unit that you won't need. Connect the unit to the computer, start Home, and move to the category for 'remove items from my unit'. You get there, if I remember, by selecting add maps/voices and arrow right till you get to the 'remove...' selection.
When you click on it, see if there is something(s) you can live without. such as voices you don't use, mp3s, photos, etc.
If you do remove items, then try the install again. If it works, fine, if not...move to choice two.
Choice two......the downloaded new map is in compressed form in your tomtom/download folder. Make a copy of this cab file and keep in another folder on your hd. Go back to the first cab folder and remove from it items you won't need, a good example will be cspeechxx.dat files for States you'll never visit or need the tomtom for. Texas, itself, is more than 40 megs.
Once you remove a variety of those files, the resultant cab file should be smaller. Try the install again. If it works, fine. If not, choice three (and probably the preferred one in my mind)....
Choice three........
Buy a sdhc card of at least 4 gigs. Install in the unit. Connect unit to computer and start Home. Home should ask whether you want internal or external selected. Select external (the sd card). Now install the map there. Your unit will still 'see' the map on the sd card.
BEFORE DOING ANYTHING.................
Make a complete Explorer,not Home, backup of your existing unit's contents. Make sure hidden folders/files are showing.