The celeron processor is fine at 3 Gigaherz but they are smaller processors - usually used in small business practises. you didn't say how much ram it has. You only need 1gig of RAM however.....
Windows7 will work fine in almost any computer but depending on what you will be using it for would determine the capabilities you will need.
If its just for minor data processing purposes (such as letter writing or using excel to make spreadsheets, etc.) or minor web surfing then its ok to have say 3gigs of RAM. (although IMO thats still too little but then I'm kinda impatient)
But for more intense purposes you really should have 4 gigs or higher of RAM. A decent graphics card - 512mg or higher with DirectX10 compatibility. Surfing the web today is much different then a few years ago. Websites have become much more interactive plus there are things you will want to watch online which require more power and speed.
Personally I wouldn't buy any computer with less than 6 gigs of RAM (or at least the capabilities of installing extra ram to meet those 6 gigs) and a cpu of 2.4 GH or higher with either a dual core or quad core processor (or higher). And 64bit encryption because 32bit is on its way out. Programs run better in 64bit encryption.
To be on the safe side, you should have a hard drive (HDD) that has at least 300gigs of space on it 500 or more is better. You would be surprised how easy it is to use up space on a pc.