Ubuntu 9.04 totally destroyed my Win7 install (even the MBR was unreadable afterwards) once, but it was because my PC had a very weird configuration (it had a mixed PATA/SATA configuration, with apparently the boostrapper for Win7 and Win7 itself being on different drives). Apart from this one case, I installed Ubuntu (up to version 10.10) several times on different computers (desktop, notebook) hosting Win7, without a problem. The tool for resizing partitions included with Ubuntu seems rock solid to me.
A Windows tool like PartitionMagic, on the other hand, failed me several times on very simple tasks, so even when I don't need to install anything, for any partition-related task I'd rather rely on a good ol' linux tool like gparted.
Conclusion: you will be fine, but save your data first (general IT motto). And your new PC came with legal Win7 key you can use with any Win7 image, in case you wonder. Just make sure you have that key and even an install failure will not cost your more than the time to reinstall.