Only two words: you can't.
This is, of course, assuming you were full of bravado and kept putting off taking regular backups. If you don't belong to this category, and nonchalantly whip out your backup media and restore everything with the click of a mouse, please curl up in a corner and die somewhere. You're not helping the non-backup-taking people at all. And yep, this goes without saying, but I'll say it anyways - wipe that smug look off your face.
One word of advice to non-backup-taking people - never play around with partitions on your HD 'just because you feel like it'. The consequences may not be very pretty.
If you should choose to disregard aforementioned advice, DO NOT UNDER ANY CIRCUMSTANCES USE THE WINDOWS DISK MANAGER WHILE ON WINDOWS XP!!
It is NOT meant to be a tool to fiddle around with partitions with data still on them. Use a real partition manager, like Symantec [erstwhile PowerQuest] PartitionMagic or Acronis Disk Director Suite. Of course, these cost money, but you can never put a price on data. [Okay, maybe you can, but I digress...] You could also use the Ranish doodad or Terabyte's BootItNG, or what the hey, even FIPS.
I used Windows Disk Manager exactly once. All I did was delete an ext2 partition (which incidentally STILL shows up as "Unknown Filesystem" - and WinXP came out MUCH MUCH later than, say, Slackware 8.0. I wonder when Microsoft will grow up and accept that competition exists and it's good to get along...) and quit the tool. The next thing I notice - My Computer no longer displays my MP3, Movie and My Documents partitions. That's 55 GB of data gone. Just like that.
After experiencing that sinking feeling for about 15 minutes and just sitting there on the verge of tears, I decide to try out an Acronis demo, and sure enough, it tells me my partition table's gone pear-shaped, and even helpfully restores it, and the partitions, along with all the data. Only, itturns out to be not-so-helpful later, when it politely informs me that the demo does only that - demo the capabilities of the full product.
That was the closest I came to blowing US Dollars 50 on software.
Googling (googlying?) around in a rather defeatist manner leads me to TestDisk, which I will continue to revere for as long as I live. In spite of its clunky fdisk-like interface, it not only set right my partition table and let me embrace my not-too-long-lost-but-all-too-precious data, it also did it in a way that PartitionMagic does NOT get anal about!
I did cry this time - tears of joy.
Never has any software made me feel so happy. When I can afford to make random donations for the sake of free software, I will do so first and foremost to the makers of TestDisk. And then to Ubuntu, but that's a different blog post altogether...