When you start your office it is very important to keep your costs low. Taking windows software will be very costly for boot strapped companies and Ubuntu is the best alternative and free to use open source operating system from linux. It will have all the tools and you will love it after using it for a while.
With no windows the biggest challenge is to create word and ppt document, open office from linux simplifies and you can use it for free with out paying a single penny. Though it may not be as user friendly as ms office but it serves the purpose.
Sending emails to customers is very important and as a startup you cannot afford costly subscriptions. Mail chimp offers free emails of 12000 every month for 2000 subscriber. It is the best tool to keep engaging your customer. Once your base increases you may upgrade plan.
Drive is other alternative to Microsoft office and helps in collaboration as well where you can together edit documents and store in cloud. Each mail account gives your storage of 10gb, nut you may have to pay 2$ per corporate email per month.
Drop box is also a simple storage solution for sharing larger files with your team members and collaborate with out any additional cost.
Communication and issue tracking is very important thing for startups. Slack is pretty good software that helps you collaborate, communicate and increase productivity for the company. You can also integrate skype, Github, drive, dropbox technically all software tool with once click integration. It ha also a mobile app like whatsapp to make communication more mobile.
Your startup needs a website to showcase your products and services. AWS from amazon provide free hosting services for startups for 750Hrs. You can technically run website on small server for entire year with out paying a buck.