It's possible that "SignupShield" came bundled with the installation of some other program or device, for example SanDisk Cruzer Flash Drive:
http://www.everythingusb.com/sandisk_cruzer_titanium_2gb.html
* Interface(s): Hi-Speed USB
* Dimensions: 2.28" x 0.75" x 0.28"
* Capacity: 2GB, 4GB, 8GB, 16GB
* Read transfer speed: 15MB/s
* Write transfer speed: 9MB/s
* Bundled software: Skype 2.0, CruzerSync, SignUp Shields, Avast! Anti-virus
* Special function(s): U3 Smart Platform
* Supported platform(s): Windows 2000 / XP
I agree with Jersey Devil & canetoad's suggestions, run Malwarebytes and any AV you have installed. Another good free malware scanner is SUPERAntiSpyware.
Malwarebytes download from FileHippo:
http://www.filehippo.com/download_malwarebytes_anti_malware/SUPERAntiSpyware download:
http://www.superantispyware.com/superantispywarefreevspro.htmlETA: SignupShield also installs as an add-on in Firefox:
https://addons.mozilla.org/en-US/firefox/addon/3476 In that case, to uninstall go to
Tools > Add-ons > Extensions > delete SignupShield.