Pre-orders of Microsoft’s latest operating system, Windows 7, have shot to the top of online sales charts, and knocked Microsoft’s European store offline.

The software giant was offering two versions of Windows 7, Home Premium and Professional, at a significant discount in the run up to its October release. Home Premium is available for around £50 and Professional is offered at around double that price. The promotion is set to end in early August.

The two editions shot to the top of online retailer Amazon’s charts, and according to a report on the BBC, “Amazon said that sales of Windows 7 in the first eight hours it was available outstripped those of Windows Vista’s entire 17 week pre-order period”.

The rush to pick up discounted copies of Windows 7 was repeated at other online resellers, including Play.com and PC World. However, Microsoft’s own store failed to cope with the demand and a few hours after the pre-orders opened the site collapsed.

“Due to the eagerly anticipated Windows 7 pre-order offer we’re experiencing a higher level of demand on our website than usual. This means you can’t access the site right now and we’re sorry about that. We’ll be back up and running as soon as we can so please try again soon to get your hands on a copy of Windows 7,” a statement on the site read.

Service was restored after a few hours.

Analyst house IDC recently estimated that Microsoft will ship 177 million units of Windows 7 by the end of 2010, with 40 million of those sales coming in 2009. Sales could be boosted by business that skipped Vista, Window 7’s disappointing predecessor, upgrading to the new version.

A recent survey by ScriptLogic discovered that six in ten businesses are planning on skipping Windows 7 – but the survey also revealed that 5.4% planned on having it in place by the end of 2009, with another 34% aiming to have Windows 7 installed by the end of 2010, which points to Windows 7 being a success for Microsoft.

A report in the Guardian points out that, “the planned adoption rate for Windows Vista was about the same, while for Windows XP actual takeup was no more than 14%”.

Windows 7 will be available for business on September 1, while consumers will have to wait until October 22 to get their hands on a copy.