Anyone buying a PC with Windows 7 pre-installed will be able to swap it for XP or Vista.
Microsoft has confirmed that the licence conditions under which the software will be sold will allow people to downgrade.
The conditions will apply to both businesses that buy licences for Windows in bulk and consumers that get the operating system on a PC or laptop.
No firm date has been given for the release of Windows 7's final version.
Downgrade rights are common in Microsoft licensing terms and conditions but the software giant has been forced to expand and extend them for XP, given user reluctance to move to Windows Vista.
Microsoft dispute claims that Vista has not been popular, quoting figures that suggest it has outsold XP over similar time frames.
However, Microsoft has twice granted Windows XP a reprieve to allow computer makers to get licences for it for far longer than was originally planned.
Windows XP, released to consumers in 2001, was also granted a lifeline to ensure that it could be used on so-called netbooks - cut-down net-capable laptops that are proving very popular.
At the same time, computer makers such as Dell and HP have been exploiting clauses in the licensing terms that let them rollback machines with Vista pre-installed to the older operating system.
The news comes as the cut-off date for free mainstream support for Windows XP ends. From 14 April, Windows XP and Home plus Office 2003 enter their "extended support" period.
This means the only updates and bug fixes these products will get will be to improve security.
Microsoft has said that the release candidate of Windows 7, which will be broadly similar to the final version, will be released in late May 2009. The final version is expected in January 2010.
Spam overwhelms e-mail messages
More than 97% of all e-mails sent over the net are unwanted, according to a Microsoft security report.
The e-mails are dominated by spam adverts for drugs, and general product pitches and often have malicious attachments.
The report found that the global ratio of infected machines was 8.6 for every 1,000 uninfected machines.
It also found that Office document attachments and PDF files were increasingly being targeted by hackers.
Microsoft said people should not panic about the high levels of unwanted e-mail.
Cliff Evans, head of security and privacy for Microsoft in the UK said "The good news is that the majority of that never hits your inbox although some will get through."
Ed Gibson, chief cyber security advisor at Microsoft, said the rise in spam was due to traditional organised crime figures moving away from exploiting software vulnerabilities and "targeting the weak link that is you and me".
"With higher capacity broadband and better OS (operating systems), and higher power computers it is easier now to send out billions of spams. Three or four years ago the capacity wasn't there."
Paul Wood, senior analyst at e-mail security firm Message Labs, said he was surprised the Microsoft figure for unwanted e-mail was so high. "Our own analysis shows that around 81% of e-mail traffic we were processing was identified as spam and unwanted," he said.
MessageLabs said spam rates had fallen at the end of 2008 as an ISP which had been hijacked to send out spam mails to users had been taken offline. "As a result of that, a number of developers in botnet technology at the end of last year were trying to regain botnet control and increase capacity and return to previous spam levels. "It won't be far off before we see return to those levels."
The report, which looked at online activity during the second half of 2008, also pinpoints the countries that are suffering from the most infections of malicious software, or malware. Russia and Brazil top the global chart of infections, followed by Turkey and Serbia and Montenegro. It said that the type of malware varied from country to country.
"As the malware ecosystem b