Hiya,
Why do you need a new modem?
If you are using Internet Explorer, click on your tools menu and then select "Internet Options" (You can also access this from the control panels.
Just change the address in that page and click on apply and then ok.
That should then set your home page.

Have been pipped at the post.... literally !...
Do you mean you want to change your "wallpaper" (the pic of your two little fiends) ?
or  Have Plusnet come up when you connect to internet instead of Google?
for second version....
Get Plusnet Home page on your screen.
Right click on the address  and then "copy"
now go to bottom left corner, click on "START"    > Control Panel > Internet Options
You will have a grey box with General tab
In the white window (entitled  To create home page......)
"paste" the url for Plusnet home page,
Then "select" it 
bottom of grey panel.... click on apply
then O.K.
Plusnet will be your "default" home page when you connect to the internet.