Following on from B Seven's comment: just go to your WAMP traybar icon, click on it and do the following:
Apache --> Alias directories --> Add an alias

This will pop up a terminal window to create the alias.

Here you can enter in the subdirectory part name e.g. the site
part in http://localhost/site
and the path to your alias directory e.g. c:/projects/site
, which you can copy from the Windows directory address bar and shift + insert into the terminal.
If you get a 403 Forbidden
error (see this question) on browsing to your aliased directory, click on that alias directory from the WAMP UI and edit it such that:
Alias /site/ "C:\projects\site/"
<Directory "C:\projects\site/">
Options Indexes FollowSymLinks MultiViews
AllowOverride all
Order allow,deny
Allow from all
</Directory>
Resembles:
Alias /site/ "C:\projects\site/"
<Directory "C:\projects\site/">
Options Indexes FollowSymLinks MultiViews
Require local
</Directory>
And you're good to go. If it opens in one line, make sure you keep it in one line or WAMP won't be able to read the alias properly and come online. (Not sure why, maybe to do with incompatible line endings or something of the sort.)