Safe to delete C:\drivers?

Personally, I wouldn't.

This folder almost certainly has various drivers that you'll need if you ever rebuild the system, and it's far easier to keep hold of them somewhere than to have to find them all again. At the very least burn them onto a CD before deleting from the HDD.

(Plus I usually find that they're fairly small folders since drivers aren't usually that big... are you really going to save that much space?)

Yup. Windows will copy them to where they're needed during installation.

Presumably this directory was created by some "helpful" third party installer.

Explanation of the Folder

The location C:\drivers is very probably the path to which the original equipment manufacturer (OEM), such as Dell, HP, etc., copied all of the hardware drivers for your computer as installed at the factory. If this is the case, each subfolder of C:\drivers that contains a relevant *.inf file will be listed in the following location in the registry: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\DevicePath. When Plug and Play detects new hardware, it searches this list of locations for drivers. The selected driver is copied somewhere under the Windows folder, but it is recopied from the original location if you reinstall a driver.

If you have a computer from a large vendor that keeps drivers available on its website for years, then you may delete this folder tree and just know that you will have to download any necessary drivers from the web in the future if needed. This could be problematic if you decide to reinstall Windows and Windows does not have a driver included that is compatible with your network card. (You need a network connection to get drivers from the Internet, but you need the drivers before your network card will work.)

Recommendation

I recommend that you do not delete this folder tree. These drivers usually do not take up very much space. Instead, start by running the Disk Cleanup Tool. This tool checks for unnecessary temporary files, and it will suggest to you ways that you may free more disk space, such as uninstalling unused programs. On Windows XP, it is located at Start|All Programs|Accessories|System Tools|Disk Cleanup.

If you still need more space, check the following locations; they usually have numerous files (some of them large) that can be deleted. The Disk Cleanup Tool may have already emptied some of these folders. (Note: Text enclosed in percent signs refers to an environment variable. To get a list of what these mean on your system, type the set command at a command prompt. Alternatively, type the environment variable with percent signs into Windows Explorer's address bar and press Enter to find the value of a particular variable.)

• %TEMP% - Usually "C:\Documents and Settings\username\Local Settings\Temp" on Windows XP
• %USERPROFILE%\Local Settings\Temporary Internet Files - %USERPROFILE% is usually "C:\Documents and Settings\username" on Windows XP. I just checked mine, and it has 15,113 files taking up 439 MB of disk space!
• %SystemRoot%\$NtUninstall* - %SystemRoot% is usually "C:\Windows" on Windows XP. WARNING: Deleting these folders removes your ability to uninstall the Windows Updates to which they refer. Because each update bases its uninstall formula (stored in one of these folders) on the state of the system when it is installed, you should delete all of these or leave all of them. Don't just delete some and leave others. I just checked mine, and it has 4,639 files taking up 577 MB of disk space!

Be sure to check the first two locations for each username's profile that is present on the machine.

As you are using a ThinkPad/Lenovo T23 computer, this folder is safe to delete. They create this folder with all the IBM drivers so that if you have problems with your display driver or something, you can re-install the "factory default" driver from that folder.

Also, when updates are downloaded, this is the default extraction folder. The folder will be re-created next time updates are downloaded and extracted.

So, yes you may delete it if you need more space.

Another significant space hog is \WINDOWS\ServicePackFiles, which contains a combined i386\ folder from any Service Packs installed on the machine. It's handy to have around in case of a rebuild (for the same reason as the \drivers folder you mention, see @cagcowboy, @Rydell), but if you're really stuck for space you can get rid of it without any immediate problems. If possible I'd go along with @cagcowboy's advice about archiving to a CD first...

Also, anything in WINDOWS\ beginning $NtUninstall... (visible after turning on Show Hidden Files (Tools>Folder Options>View)) can go, if you don't need to uninstall the system updates that they refer to.