If you suspect a file to be malicious, you can send it to Microsoft for analysis. The Microsoft Research and Response will check the file for malware, which may include viruses, spyware, worms, trojans, rootkits, adware, etc.
Where to submit malware & suspicious files to Microsoft
You can submit the file directly to Microsoft using this webpage at the Malware Protection Center.
It is always recommended that you zip the file and send it in compressed form.
Where to submit virus & malware files to antivirus companies
If you want to submit virus & malware files to antivirus companies, here are a few links which will help you:
If you know of any more, please do share!
TIP: X-ray a freeware that automates the process of submission of malware files to over 30 security companies.